The generic sign for in is signed by making your non-dominant hand into a cup shape, then with your dominant hand's fingers touching, place your fingers inside the cup.
If you are signing in something specific, you would incorporate that into the sign. For example, if you were signing, "in a drawer," you would make a motion as if you were opening a drawer, then move your dominant hand into the imaginary drawer. This more specific signing is of course best left to more advanced signers.
You can teach the signs for in and out together. Demonstrate each by placing an object in a box, and then taking it out.
This sign is very easy to teach. Give your baby a container and a little object or toy and have your baby put the object in or out.