To do the baby sign for apple, make your hand into a fist, then extend the index finger knuckle (your index finger is the one next to your thumb). Bring the knuckle to your cheek and twist it about. Imagine your cheek as an apple, and you are twisting off the stalk.
Apple is quite a complex sign, and you will see this simplified a lot at first. Often, the baby will just place their fingers on their cheek, or twist their hand near their cheek.
We mainly use apple as a food sign. Our baby loves cut-up pieces of apple, so I will do the apple sign before and during eating slices of apple. I will say, "Do you want some apple?" as I put the apple in front of her. Then I reinforce it as she eats by saying something like, "Mmm, apple."
We make a game out of doing signs for various foods. Beware! Do the signs just before meal times, because this game is sure to make both you and your baby hungry! You can also teach the sign for apple when you're out with your baby at the grocery or farmer's market.