If you want to do it in two steps with the same die, first you adjust your die to seat without crimping - with the die not fully seated, place a case in the shellholder and raise the ram completely and then turn the die in until you can just feel it touch the top of the case. Back off 1.5-2 turns, reset your lock ring (or use Donkee's washer trick), then set your bullet seater stem where you want it and seat the batch. I like to seat cannelured bullets to where the top of the brass is very close to the top of the cannelure, so there's plenty of room for the crimp.
When it's time to crimp the batch, back off your seater stem 5-6 turns so it cannot possibly contact the previously seated bullets, then put another case in the shellholder as before and raise it completely. Loosen the lock ring on the die (or remove washer) and turn it down until you feel it touch the top of your case. Lower the ram and turn the die 1/2 turn or so and then run the case back into it. If you like that crimp, set the lock ring and go. If you need more, I'd add 1/4 turn at a time until the proper crimp is achieved. Overcrimping is very common - you generally only need the lip of the case to be rolled into the cannelure.
I've never had a problem seating and crimping in the same step myself - but I also tend to keep my brass grouped by headstamp so it's mostly the same length. I even trim my premium magnum revolver brass for consistency and I always readjust my die for each batch. The main key is timing - the crimping ring in the die cannot start touching the lip of the case until it's cleared the bottom edge of the cannelure - otherwise it jams into the bullet and the case gets buckled as the bullet continues down. If your cases are of inconsistent lengths, you'll drive yourself mad as your results will vary between no crimp and overcrimped, buckled cases. The compounding issue is most loaders' tendency to overcrimp.
