From the map, pick out a green signpost near where you want to fast travel, and press 'E' again. Press 'E' and the screen changes to the map. If you are at the signpost, you will notice an 'E' above it. You must physically be by the signpost (in game) to fast travel.

Originally posted by rzn6jw:Signposts (that you've already found) are marked green on your map.