I make up 4 bottles in the morning, boil kettle, fill sterilised bottles, add powder to boiling water, swirl to mix (not shake as this puts lots of bubbles into it and causes wind) then cool rapidly in a tray of cold water with ice blocks in - keep in fridge til needed - then heat up. I then make a further 4 in the evening for the night shift - same way.
I did this with my son and never had a problem........