Bonus post: How to change a micro tank or small bowl.

I don't currently have a micro tank or a betta bowl, but I have a buttload of experience keeping betta bowls maintained (thank you, Craft Warehouse, F you, Craft Warehouse, stop selling fish. They're not craft supplies, and they're not just "product and numbers" in the words of my manager, the words that made me quit that job).
 
Big big big, important note: different fish have different needs. It is possible to keep a betta fish in a bowl larger than a gallon without negatively impacting their life. It is not possible to keep a goldfish in a bowl without making the fish really sick and cutting its lifespan in half. This post does not apply to any fish other than betta fish.

You're going to run into people who say you can't keep betta fish in bowls. Most of them have never tried, and they're just parroting information they hear on the internet. I, personally, get more enjoyment out of watching fish who have swimming space and a low-stress life, so I keep my fish in larger tanks. However, if you've decided that you want to keep a fish in a bowl, I think it's important to have some useful and correct information out there.
The first thing I want to say, though, is that I have maintained a lot of fish in a lot of fish in bowls ranging from .5 to 2.5 gallons, and I do not think it is possible to meet a betta fish's survival needs in a bowl or tank smaller than one gallon. The difference in fish health between living in 1.5 gallons versus living in 0.75 gallons is easily visible.
If you've already got a 0.25 gallon betta cube and you want to keep something in it, look into planaria, the only pet you can cut in half to make two of them. Other great pets for a betta cube include m&m's and plastic dinosaurs.

I'm going to be kind of harsh here. In a bigger tank, you get a little more flexibility with what you want to do, and you get a little more flexibility with how you want to care for it. When you've moved your fish down to the smallest container possible for it to stay alive, you need to be ready to provide everything you need to keep it alive.

First, if you're doing a betta bowl or a micro tank (tank under 2.5-3 gallons), you need certain things when you set it up:

1. A bowl that is as big as you can physically pick up and move around when it's full. It needs to hold a gallon or larger, or it's too small for the fish. If it's too heavy for you to move, you're never going to clean it on a regular schedule. If you can't find a bowl that's over a gallon, but light enough for you to move, either get a lighter bowl and a lighter decor, or get a 5-gallon and a filter. I cannot stress enough that it is actually much easier to maintain a 5-gallon tank than it is to maintain a small bowl. Less stress on you, less stress on your fish.
2. A heater that you trust. Before you buy a heater, look up reviews of it. There's some heaters marketed especially for bowls that overheat and kill fish. The less water you have, the more quickly an overheat will kill someone. The basic rule is the less water you have, the less room you have for things to go wrong. Your heater needs to keep the tank at roughly 78 degrees. A small bowl or tank is going to be colder than the room temperature, and very likely to get too cold if you turn the heating off or the air conditioner on. You can't let your room regulate your temperature. You need to heat it yourself.
3. NO FILTER. That's right. Unless it's three gallons or larger, no filter. Look back to #2 and read that bold text again. Betta fish have an adaptation that allows them to live in a higher ammonia environment than most other fish. They don't have that same resistance to nitrite. Nitrite is also, on its own, more toxic than ammonia. If you don't have enough water in your tank, your cycle will crash, and it will make nitrite. Among the many things that a filter does, it makes it easier for you to take care of a mini ecosystem in your tank. If you want a micro tank or a betta bowl, you need to be ready to change the water once or twice a week. Once again, a bowl is harder to maintain than a 3-5 gallon tank. 
4. Easy to remove, easy to clean substrate. The poop goes into the gravel, and you're going to have to get it out. My favorite substrate for a bowl is glass rocks or marbles. You want the ones that are the diameter of a quarter, or smaller. If you want big rocks, you need small rocks to fill in the gaps, because for some reason betta really like to get stuck under rocks. They like getting stuck. They don't really like the subsequent death. Whatever rocks you use, you need to be able to take them all the way out and wash them off, and wash the bowl out. You can use gravel and a strainer, if you want. You can also have just a bare bottom of your tank. That's all up to you, but you need to be able to remove all of your substrate and wash it. Sand is really bad for betta bowls.
5. Enough places for your betta to hide. Being in a small, clear bowl does not make your fish feel any less exposed than being in a large, empty tank. Betta need some form of cover, or they get stressed out and more vulnerable to sickness. You need a cave they can't get stuck in Flower pots that are unglazed or glazed with non-metallic high-fire glaze work well if you lay them on their sides. Plants that can't scratch them and that won't rot in the water are a good thing, too. I like weighted silk plants.
6. Nothing else polluting their water. Live plants are pretty, but live plants in a bowl do not filter the bowl. Live plants in your betta bowl may make the conditions in the bowl much worse. Plants do not filter your water. PLANTS DO NOT FILTER YOUR WATER. Lucky bamboo and peace lilies are both land plants, and they need to have their leaves exposed to the air. All live plants can have parts that die, break off, and rot. When you clean your bowl or microtank, you're going to take everything that's in it, out. This can stress out the plant and make roots break off, or just make your plant die. Dead plant bits are just adding to the shit you have to clean out of the tank. 
7. Food and respiration space. Betta fish need to take gulps of air from the surface of their water. It feeds their labyrinth organ, which is sort of like a lung. This is the adaptation that helps them live in higher ammonia environments than some other fish. If they can't get air from the surface, they die. The surface of their water needs to be mostly clear, with space to allow oxygen to reach the surface. This means that, while pretty, putting a peace lily on top of a betta bowl is a bad idea. Also, betta fish don't live off the roots of the peace lilies in the plants. They're carnivores. If you've seen a betta in a peace-lily-on-top setup biting at the roots, it's usually because it's starving to death and willing to try anything. A betta's body is unable to eat and get nutrients from plant roots in the same way our body isn't able to eat and get nutrients from plastic soda bottles. You need to give your fish actual food made from actual meat.

Setting up some of this might be inconvenient and some of this might be hard or expensive. That's really tough luck for you, there. If you've been informed about the basic needs of your fish and decided that not going to meet the basic needs of your fish, you don't deserve a fish. You don't deserve a pet. They're not a right, and if you can't take care of yours and you can't find someone who will, it's a privilege that you don't get.
 
So, you've got your bowl set up? Good. Now, it's time to clean it.

• Grab your ammonia test kit (YOU NEED AN AMMONIA TEST KIT. It really doesn't matter if it's expensive and you don't want to spend that much money. Trying to maintain a bowl without owning an ammonia test kit, "because it's expensive," is like trying to store food in a refrigerator that isn't plugged in "because electricy costs a lot." Someone's going to get sick because of what you're doing and they're going to die). Do a test on your water. If your ammonia is above .5ppm, you know that it's been too long since your last water change. You need to do a water change when your ammonia goes above 0.5 OR when you see crap accumulating on the bottom OR once a week, whichever comes first. After a while of testing, you're going to have a good idea of when you need to change your water, but you should still do a test before each change. As your fish grows up, gets healthier, gets sicker, water gets warmer, water gets cooler, the ammonia production will change.
  A good rule to go by: If you tested your water and it was 1ppm, and you haven't done a change for 12 days, you should change your water and check it again in 6 days. If you test your water 6 days later and your ammonia is below 0.5ppm, you need to do your next change in 6-7 days. Your ammonia test will give you an exact answer for the question of how many days should you go between changes. 
• Cup your fish. You can use the cup you got him in, or any other clean cup that you save for that purpose. Get him out of the bowl and put him somewhere where you can keep an eye on him, won't knock him over, and where the cat won't see him.
• Unplug your heater. You can let it cool for about half an hour, or grab another cup full of water and stick the heater in that to cool off while you clean the bowl.
• Take the whole bowl to the sink of your choice. Scoop out some water first, if you need to. You can also use the bathtub if you don't like using a food sink. You can use your garden hose outside if you don't like using your bathtub.
• Pull out your castles and plants and other things and set them aside.
• Use a bucket or a large bowl or a colander that you use only for fish, and pull all your substrate out. Try to get as much rocks and as little water as you can. Dump the rest of the water out of the bowl. 
• Do not use soap on anything at all. No soap. Don't even wash your hands with soap until you've finished the whole change and made sure your fish buddy's okay.
• Rinse out the bowl. Using a paper towel or your hand, wipe the inside of the bowl to get any extra gunk out, and then rinse it again. 
• Rinse off the rocks and all your decoration. Grab your now-cooled heater and rinse that off as well.
• Fill the bowl halfway with water that's roughly the same temperature as the temperature in the cup your fish is in. If your heater goes under your rocks, put it in place. Don't just dump all the rocks into an empty glass bowl, or you can break it. You can do it a handful at a time, or fill up the bowl some of the way before dumping the rocks in. 
• Either decorate the bowl the way you usually do, or take the bowl to where it lives, and then decorate it.
• Fill the bowl up all the way. Match the temperature of the water to the temperature of the cup your fish is in. Put conditioner into your water. (I like to fill the bowl with tap water, and then put the conditioner in. I like to pretend that the chlorine in the water kills some of the remaining bacteria in the bowl. I objectively know it doesn't, but it makes me feel good and it doesn't hurt anything)
• Put your fingers over the top of his cup like a screen, and dump all the water out of the cup without dumping the fish out. Immediately dump the fish into the bowl. This makes sure that you don't add high-ammonia water back into the bowl.
• Look at your fish and tell him he's pretty. Watch for a couple minutes to make sure he's breathing okay. If his breathing is very labored and doesn't settle down after two minutes, take him out and figure out how a toxin could get into his water. Change all the water again.

Can you keep a betta fish healthy in a bowl? Should you do it? That's really up to you, and up to your fish. If your fish is sick in these conditions, then you need to be ready to move him. See above about fish are not a right.

And there you go. Sorry for no pictures, but I'm not at a point in my life right now where my lifestyle gives me enough free time to maintain a high-maintenance thing like a micro tank, so I couldn't take any.

Take care of your fish, and they'll make you happy.

😃