I think that the best way to do it is to sterilise your bottles whilst boiling the kettle. Then when the kettle is boiled let it cool for the maximum time of 30 mins before making up enough bottles for 24 hours with the milk aswell. Then put all the bottles in to a bowl of cold water for about 20 mins to cool them down and then store them in the fridge. When you need one just stick it in the microwave for about 30 seconds (shake it well after to get rid of hot spots and check the temperature) or put it in a bowl of boiling water to heat it up. The other way to do it is to make up the bottles with boiling water and let them cool(no need to refridgerate just water). When you need them heat them up and then add the milk. If you make them with just water heat them up before adding the milk. The formula isn't sterile so it needs to be added to boiling water to kill any germs especially whilst your baby is newborn.