LeeinVA1 gave the quick steps of one way to follow the GPS back to your original waypoint. The manual will help step you through the specifics of which buttons to push, when. It does take some practice BEFORE you need it for real!
Another way to find your way back is to follow the"trail"it shows you followed on the way out and retrace the trail. You'll need to practice zooming in and zooming out some to get the best view of following the trail.
I've also set the waypoint and straight line trail between (for boundary property lines) and followed the trail it shows. It'll keep you within 10-20 feet either side of the trail as long as you keep somewhat of an overhead to the sky open. It needs to keep communications with the GPS satellites ongoing. Depending on how many satellites it picks up and which positions in the sky, the accuracy can change as you go, but you'll stay within sight of the actual virtual trail most of the time. Only if you run into unnavigateable obsticales might you need to go around and temporarily leave a pre-set trail.
Have fun. Always bring a compass as a backup, in case the thing stops working for you or you aren't sure. It's a good way to verify even with a GPS.