I always prepare fresh bottles. When Omar was tiny, I used to add 1 oz of boiled water to the bottle then add all the formula to it to sterilise it, then top up with cool water (pre-boiled), give it a stir & ready to feed. You can prepare it the same way & keep it in the fridge (it lasts for 24 hrs in the fridge), then run it under hot water to warm it & give it to your LO. Or you can boil a kettle, leave the water for 15-20 mins to cool down (it should be around 70 oC to sterilise the formual), add it to the bottle add formula, shake/stir. leave it to cool down a bit & into the fridge, then heat it when needed. But I never pre-prepared bottles.