If you've HAVEN'T yet launched the game, you can just go on ahead and link your Steam to your old profile directly:

Step 1. Log into your old Haven account, then go to the profile page.





Step 2. Go to "Account security", and elevate your login.





Step 3. Click that "Link your steam account" button. Done!

If you've launched the game, Steam should already have made an account for you that you'll have to unlink.

Step 1. Go to the log-in page on the Haven & Hearth website. Then, click as indicated to make your unnamed steam account into a Haven account.



Step 2. Follow the steps of the earlier section, except unlink the steam account on this new account instead.

Step 3. Log into your old account, then follow the steps on that prior section once again to link your Steam account to your old account.

WARNING. As of this guide's date of writing, you can't transfer your characters out of that Steam account you made by accident, so, if your old account was verified or subscribed, DO THIS AS SOON AS POSSIBLE so you don't lose progress.

Simple, right?