1) Visit https://accounts.klei.com/ and sign in using your Epic Games account (that owns Griftlands) - if you have not created a Klei account yet, please do so and verify your email address.
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) Once signed in to the Klei accounts website using your Epic Games login, click on the “My Games” option in your “Settings” tab on your Klei Accounts page, and ensure that Griftlands is noted as “Purchased” on Epic. 
4) Next, go back to the “Settings” tab and click on “Merge An Account” to allow your Klei account to add in/merge your Epic and Steam ownership for Griftlands. Click on the Epic Games + Steam Logo image to begin the process. (Please note that you can only do this process ONCE; ensure that you are logging in using the Steam account you really want Griftlands on.) 
5) On the next page, read the information completely before putting any checkmarks in the checkboxes. If instructions are clear and understood, follow the steps on the page to continue the merge process. 
6) On the next page, Steam will ask for your sign-in, log in using the Steam account you would like to receive Griftlands on. (Please note that you can only do this process ONCE; please be sure that you are using the correct Steam account.)
7) If the sign-ins for both your Klei Accounts (via Steam and Epic) are successful, you should see the page as shown below. Click on the “I Understand, merge these accounts” button to finish the process.
If you encounter an error message such as the ones in the picture below, ensure that the email address used in your Steam and Epic - Klei Accounts match. Click on the “Fix Warnings” button to fix these issues.
If the merge was successful, you should see the page as shown below. Log in to your Klei Account once more. 
8) Finally, click on the Settings Tab and choose “Game Ownership” once more and you should now see a “Grant Game” button on the Status column for Griftlands on Steam. Click on the Grant Game button to get Griftlands on your Steam account. (Please note that this process may take a few minutes to complete.)
You now have Griftlands on Steam! Enjoy.