How do you warm up breast milk?

Mel

I recently gave birth to my beautiful son I decided to breastfed my son. Today I had earns to run and knew that he would be staying home with someone watching him. I pumped ahead so I made sure that there would be enough for the whole time I was out. I made it home just in time for him to get fed but his babysitter was warming up his milk already. We both checked the temperature before giving it to him and it seemed fine. So during his feeding and him taking little breaks the milk gotten cold and he didn't want any more so I had to take out my boob and finishing feeding him. This is my first child is there any one out there who knows how long I should warm the pre-pumped breast milk or is it too early to try to breast fed with my breast and a bottle. He is a week old today.