http://static.garmin.com/pumac/eTrex_10-20-30_OM_EN.pdf

You can download the manual above.
However once you've got waypoints, you're halfway home (sorry about pun)

I mostly use "where to" and select the waypoint (from recent finds or waypoints list), then go back to the main menu and go to "compass" rose page as it has the compass, and speed & time info..

Occasionally I'll go to the Map page to view my track in relation to all the waypoints. All the uploaded/entered waypoints make a kind of basic map, and my previous tracks must be water deep enough to sail in. Especially if the waypoints are headlands, lighthouses, etc.

Often I use the Sun & Moon page on the main menu, to work out sunrise/set and moonrise/set and phase.
That's about all I do with the eTrex. If I sail at night I will use the power cord and have the screen on some level of brightness with the Compass rose page, as the glowing screen is somewhat reassuring on overnight trips.