1) Set up the build environment according to the techbase wiki page. This needs some thoughts and a bit of free space (measuring in GB!!!).
2) If you're ready just emerge one of the three packages "kdeedu", "marble-kde" or "marble-qt" e.g.
emerge marble-kdeThe emerge scripts will take care of all the dependencies for you. If you only need the qt-only version, just use the "marble-qt" package.
3) Look at Marble, look into the far remotest parts of the world and of course have a look at the OpenStreetMap maps in Marble (go into the Map View part on the right side and switch to OpenStreetMap).