I have always introduced a crupper with the horse either on a longe line or (even better) in a round pen (and this is assuming the horse lets you handle its tail and hindquarters without issue already).
After working the horse enough to make sure I have their brain, I will run an extra length of longe line from a baling twine cheater around a surcingle D to under their tail and back to the surcingle on the other side (again with a cheater of baling twine attached to the D). It's not real tight, but not flapping either. We then get back to work.
Some horses just clamp their tails and look perplexed, then eventually relax. Others do try to remove the offending line with some bucking.. as long as they are not out of control, I always just ignored the bucking and asked them to work. We go until they relax, then done for the day.
Once they are going with the line like its no big deal (could take one or many sessions), I will switch to the real crupper. Some horses don't care, others take offense to the stiffer material and need to be schooled thru it. Next step would be to longe on some terrain (like a field with a hill) if the horse was unhappy with the process or just hop on and hit the trails if they weren't.
There is no joy equal to that found on the back of a horse.