How do y'all warm up your bottle? My LO gets a mixture of breastmilk and formula every feeding. I usually have 8oz of formula already mixed for 12hrs (I boil my water).

I have been trying to be good and use the bottle warmer and I have been following the instructions-- well I just tried to feed my LO and the milk got so hot. Like literally so hot when I tried to test it on my arm. 🙄🙄 I also tried taking it out earlier than it told me to and it was freaking cold. What the heck.