A short guide on how to install SoaSE maps and play them:

[Step 1]

Locate where the "Ironclad Games" folder is, it is usually found in Documents\My Games.

Example:
C:\Users\USER NAME\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion

[Step 2]

You should see a folder called GalaxyForge in the directory, open that folder and now there should be a few .galaxy files already there.
Now copy/paste or move the desired map files into the GalaxyForge folder.

Example:
C:\Users\USER NAME\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\GalaxyForge

[Step 3]

Once you've placed the files there they should now be accessible in-game. When you start up the game and go to the Start New Game menu and go through the map choices, the installed maps should be visible under the Galaxy Forge category. Depending on what versions of the map you've installed they should all be visible there along with all the other maps you've put there. You can access the Galaxy Forge map-category the quickest way by right-clicking once with the mouse on the Change Maps button to go backwards in the categories and you'll immediately get access to the Galaxy Forge maps.