Not to sound like an ass but read the instructions that came with the dies.
I assume you have a 3 dies set?
The thrid step will seat and crimp. Crimp basically "un-bells" the case. It takes the top of the brass and pushed it inward to hold onto the bullet. Seating the bullet just does length. You can seat a bullet and crimp and the same time. The plunger that screws in the top of the die controls length while screwing the whole die body in and out of the press controls the crimp amount.
Gun control theory - A woman raped and strangled is morally superior to a woman with a smoking gun and a dead rapist at her feet.