Getting formula preparation right matters because powdered formula is not sterile - it can contain bacteria that cause serious illness in young babies. The NHS guidance is specific, and the reason for each step is to kill potential bacteria while preserving nutrients.
Step by step
Boil fresh tap water in a kettle. Let it cool for no more than 30 minutes - the water needs to be at least 70 degrees C to kill bacteria in the powder. Pour the water into a sterilised bottle first, then add the powder. Never the other way around, as this changes the concentration.
Cooling and storage
Cool the bottle quickly by holding it under cold running water. Test the temperature on the inside of your wrist - it should feel warm, not hot. Use within two hours. Never reheat formula that's already been warmed. Discard any formula your baby hasn't finished within two hours.
Sterilising
All feeding equipment must be sterilised until your baby is at least 12 months old. You can use a steam steriliser, cold water sterilising solution, or boiling in a pan for at least 10 minutes. Make sure bottles and teats are thoroughly cleaned with hot soapy water and a bottle brush before sterilising.
Common mistakes
Making bottles in advance and storing them at room temperature. Adding extra scoops "to fill them up" - this is dangerous as it changes the concentration. Using water that's cooled below 70 degrees C. Using bottled mineral water (it's not suitable for babies due to sodium content).