As simple as 1, 2, 3!
These 3 factors would affect the bettas' growth and development.
We use 3-liter tanks to raise our bettas from the moment they are separated from their community tank. Note, the smaller the tank is, the more frequent you have to change their water.
We recommend changing your bettas' water every 3 - 5 days to prevent ammonia build up. Percentage of water change per interval would depend on your tank size. 30-50% change is normal.
Remember to store your water with water conditioner (Anti-chlorine) before using.
We feed a variety of food to our bettas. For betta fry under 1 month old, we feed a mixture of brine shrimp and daphnia. Adult bettas are fed with pellets. You can choose to feed frozen bloodworms or live tubifex worms (ensure you clean them well). Recommended to feed them twice a day.
Similarly to us, a good 15-20 minutes of daily exercise keeps them healthy and active. A mirror or another betta shown would get them flaring up!