1) You will need to merge your Klei/Epic account with your Klei/Steam account in order to redeem Griftlands on your Steam account. Please follow the steps in our support documentation/guide to merge the accounts together.
2) Launch your Epic Games Launcher, log in to your Epic Games account and start Griftlands, this will contact and inform our servers that the account owns the game. 
3) On your merged Klei Account, click on the "My Games" option in your account's "Settings" tab, and ensure that Griftlands is noted as "Purchased" on Epic. 
4) If the account merge process completes successfully, you should now see a "Grant Game" button on the Status column for Griftlands on Steam. Click on the Grant Game button to claim Griftlands on your Steam account. Please note that this process may take a few minutes to complete. 
You now have Griftlands on Steam! Enjoy.