Usage patterns
〈物〉にミルクを入れるto add milk to something (coffee, tea, etc.)
I add milk to my coffee and drink it.
〈人〉にミルクをあげるto give (a baby) milk/formula
I give the baby formula every three hours.
ミルクを温めるto warm up (baby's) milk
Since she cried in the middle of the night, I warmed up her formula.