The formula companies will usually say never premake formula. But honestly.. its not reasonable.
When you are tired, baby is screaming and hungry, and you have 50 bajillion other things to do.... its so not feasible to boil water, sterilize a bottle, make your formula, etc 8+ times a day. SO here is what I do, lol.
It depends if you use powder or concentrate, IMHO. In the beginning, we used pre-mixed (really expensive omg) because we were too nervous to premake our own. Then he ended up on premixed alimentum for a few weeks anyway... But around 6 weeks he was back on regular formula. I would boil water, let cool, pour into bottles for each amount, then store in the fridge for up to 24 hours. Then add the powder as needed and heat in a bottle warmer. In Canada, they tell us to add the powder to cooled water, but I have heard in the US they say use hot water... so I'm not sure which is right! The powder can in very rare cases contain a bacteria that needs to be killed but supposedly concentrate doesn't have that risk. And for some reason, my son gets worse gas with the powder but he has a really sensitive system.
Then around 10 weeks, I realized concentrate is actually the same price here (or maybe $2 more), which is great. So now I boil the water, let cool, pour it in the formula pitcher/mixer with the concentrate and store in the fridge for 12-24 hours. When he's at family's houses, we premake them right in the bottles, and they are always used within 24 hours anyway, usually like 12 hours. I also keep all my empty sterilized bottles in the fridge just because I don't want to leave them sitting with water droplets in them just in case but I think its unnecessary technically.
When he turns 4 months, I will probably just store all the bottles in a sealed container in the cupboard after sterilizing with the caps on. Here they say its ok to stop sterilizing at 4 months, but I will still be doing it til he's around a year since we have a microwave sterilizer so its really fast (it was $40 but worth it!). Though he has started putting EVERYTHING in his mouth anyway. I never stop cleaning lately, lol... sigh.
I really would say avoid the microwave for heating formula, but some moms don't. I think its personal preference but I say better safe than sorry. For me, the last thing I'd want is to be overtired and have to deal with having to cool down a bottle that I accidently heated too long or something. The hot spots can really hide in there too apparently so I feel more comfortable with a warmer! Mine is Munchkin brand, around $30 from Walmart, takes about 1:30 or 2 mins to heat a full 8oz bottle, its been awesome. He also has a First Years warmer at grandma's house, also works great!