To sign cup, take your dominant hand and wrap your fingers round into a 'C' shape, as if you are holding a cup. Take your non-dominant hand and lay it flat, palm facing up, as if it is a table top. Take the dominant hand and place it down on the flat hand.
Imagine you are placing a cup down on a table.
If your baby has their own special cup, associate the sign with the cup. "Jamie, here is your cup."
If you have a cup handy, use the real cup as well as the flash card when teaching your baby the sign. The flash cards are great, but the real cup creates an even more vivid mark in your baby’s mind.