I just hold the rat by the scruff with hemostats and hover the rat's nose right next to the snake's face for a few moments and they take it.
Make sure it's hot and won't cool down too fast. I usually dunk them in hot water and towel them off, but rodents thawed in warm water in a baggie retain heat better than ones thawed in the fridge in my experience. All of my snakes on ft switched in about three feeds. My two adult males absolutely refuse though.