How Threadless Jewelry Works
When you purchase threadless (also called pushpin) jewelry it may or may not come pre-bent. Usually if it is sold as a 2 pc combo there is already a bend in the pin top, however, it still needs to be checked to ensure it is a tight enough fit.
When you buy a separate top and base you almost surely will have to pre-bend your top pin.
Either way here is a helpful video that explains how but be warned you must be gentle and always apply pressure from the middle of the top not an end particularly if it is a long design. You must be gentle it is easy to break them if you bend repeatedly or too aggressively and this is not returnable for this reason.