David built this A frame with a plumb line:
OK I thought it was going to be easy to describe how you use the A frame to mark the contour lines of the land....but it's actually difficult to put into words. So I'm including a couple videos that do a decent job of showing and explaining it.
Check out these videos on how to use an A Frame...we haven't used ours yet.
Using the A frame to map out contour lines on the land.
Here's another version of using an A frame: