You can set them on something or lean into a tree/fence. When laying prone you can just set it on your scope and hit the target. If you are in a situation like you mention where you know the landmark ranges then you can just use them as a field sketch and hold or dial accordingly and take the shot. Nothing wrong with setting up a range card field sketch ahead of time and using it. If you use a log book then you can use a card like the one below and make the sketch but even a piece of paper will work.

http://www.impactdatabooks.com/Snipe..._p/mls-src.htm