Fairly straightforward. After unlocking the north side of the map upon completing act 2 by going through the Jaws, you can drive to Pink Eyes stronghold. Upon arriving, you will need to destroy a few catapults outside before making your way inside to finish off the last of the attackers. After that, you have access to the stronghold. The easiest two to build, which you can do right away, is the small and medium scrap piles, requiring 1500 scrap total.