1. Start GC3
2. Click on "New Game"
3. In the lower left hand side, below the scrolling window with all the mini pictures of your currently-loaded races, there's a button labled "Steam Workshop"
4. This opens up the Steam browser and takes you to the Factions web page, where you can browse for Races.
5. Click on a Race's picture, and that will take you to the page describing the race.
6. If you like it, click the Subscribe button.
7. Browse for more Races. Once you are done, exit the Steam browser.
8. Your new races should appear in the choose immediately.
Same process to get custom-done ships.