Before you feed your baby you will need to have the bottle of breast milk or formula at the temperature your baby prefers. This can even be room temperature. It is important that you do not microwave bottles, no matter what is in them, as this will cause hot spots that can burn your baby, even though you may have tested the liquid.
You will need to use either a store bought bottle warmer for your car or home or use the old fashioned, but super effective water method. To use the water method simply take a large cup or bowl and run hot tap water into it. Place the bottle into the water and reduce the flow of water to a trickle so that the bottle is quickly brought to a comfortable temperature.
Before you feed the baby any fluid that has been warmed, check the fluid by placing some on your forearm. It is also important to note that not all babies need warm fluids. Your baby will let you know what temperature s/he likes.