Well we use ready made if out and about and A will actually take it at room temperature, so no need to heat it, which is great. You could try that first to see if DD will take it like that. If you do need to heat it then yes stand it in boiling water, a flask to take out is handy for this. Tomee Tippie do one with a lid that you can fit a bottle in which is great. You could buy a bottle warmer for at home, which does the same job really, but probably not worth it now for the amount of time you will use it.