You can make bottles up a bit in advance but they must be used that day. Store in the fridge. I found it easier to boil water in the morning and then keep it (just in a water bottle on the counter) to make up bottles when needed. In the beginning when you are demand feeding there is nothing worse than trying to cool just boiled water while your baby screams his lungs out. Reduces you to tears! You heat a bottle by putting it in hot water. No microwaves as they heat unevenly and it's very easy to make it too hot! Although I was mean, my babies got room temp bottles from quite early as waiting for a bottle to heat up in the middle of the night with a yelling baby wasnt fun! Don't shake as that puts air into it which gives baby a sore stomach, winding is hard enough without adding more air! Just kinda swirl it around. To test the temp shake a few drops onto your wrist, should be luke warm. You always put the water in first, then the powder. If you put the powder first you will use too little water and that makes it too rich, gives you a sick or constipated baby. Hope that helps!