K.P.
Show her... show her again... show her again.
There's no "trick", it's just practice. We have a Melissa & Doug "fasteners" board that someone gave us years ago and it's great! It has a zipper, buttons, snaps and a toggle (I think),so it's practicing without the stress of also getting dressed.
My son will be 5 in late May and just mastered the snap a few months ago and is pretty good (now) with large buttons and zippers. Really, it was just practice and allowing enough time for him to do it himself that made it work.