Similar to the half pass, the side, or full, pass is a type of lateral movement. In contrast to the half pass, however, the side pass does not have forward movement. You can consider a side pass to be a half pass without any forward motion. Therefore, you will start the side pass from the halt and ask your pony to yield to your leg/whip without moving forward.
To perform a side pass to the left, first halt your pony. Once at the halt, flick your whip on the pony's right side (your whip will emulate the action of a rider's leg) and maintain sufficient rein contact to contain any forward movement. Your pony should move sideways to the left, yielding to the leg pressure on his right. Moreover, the pony should remain facing forward, similar to the half pass, while moving sideways.
A couple common problems you may encounter when asking your pony to execute a side pass:
As with everything else, practice is critical. Don't overdo it though: mix things up and keep it fun!