If I have a goal for what I want to feed an animal and it's an appropriate feeder size at that time:

I just offer it and see what happens.

My first ball python ever was raised on live mice for the first few months after hatching. I took her home, and for the next three feedings I duplicated what she was used to. Then the next feeding I offered a F/T rat of whatever size (probably a small rat), and she took it. It can be that easy, just make sure you follow all the generic tips for a successful feeding of potentially tricky snakes:

1) Feed just after dark
2) Feed prey warmed above room temp so that their heat pits can pick it up
3) Feed using tongs to separate your body temp from that of the feeder
4) Don't use excessive motion on your part which may make a snake hesitant to make itself vulnerable to a big predator

I have only had one animal I've taken multiple steps with, and that was a picky hognose (which are not exclusively rodent-feeders in the wild).