Handle them every day for short periods of time. Let them smell you. Do not make sudden movements and when they want to get away from you, let them. It will take time when you get them as adults. Taming younger rats (just weaned) is a lot easier. But again, with adults, just give them plenty of time and take it easy.

I treat my breeders as pets. Of course, some of them I cannot pick up and handle like a tame rat. I have to pick them up by their tails. Others I can reach right in and pick them up with no problems.