First timers often crimp the plastic part instead of the metal. That’s like putting a hat on a hat. Useless and confusing. The crimp goes on the metal tube, not the colorful collar.
Another classic: using a ferrule on solid wire. Don’t. Solid wire is already tough. Ferrules are for stranded wires that act like tangled spaghetti. Use a ferrule on solid wire, and it’ll just wobble off, embarrassed.
How To Use Cable Ferrules at Warren Short blog