1. Go here to create your Green Man Gaming account
2. Go here and enter your details in the panel on the right.
3. Link your Steam account on Playfire (if you haven't already)
If you already have an account with Green Man Gaming, simply enter the details of your existing account in the box opposite to claim your free game.
While you're there check out the winter games sale:
Green Man Gaming Winter Sale - 20% off coupon: XMAS20-PERGMG-SVINGS
Green Man Gaming Best of 2015 Sale - 20% off coupon: XMAS20-PERGMG-SVINGS