The first batch of keys are now available to be claimed.
The game will not unlock until the 19th, but you can claim a key starting now! To claim your key, you need to access your account's unique download page for the Patron Edition of the game. There are two ways to do this.
Method 1
Go to https://itch.io/my-purchases and find Succubus Stories (Patron Edition) in your list of games. Click the "Download" button to go to your download page for the game.
Method 2
Go to https://outsiderartisan.itch.io/succubus-stories-patron/patreon-access and click "Connect with Patreon." Log in via Patreon and you will be redirected to your download page for the game.
Claim your key
Once you're on the download page, you'll find the option "Get Steam key" at the very bottom. Clicking this will claim and reveal a Steam key, which you can copy/paste into Steam.
Go to Steam and enter the key
Go to the appropriate page on the Steam website or client to register the key: https://store.steampowered.com/account/registerkey
That's it! You'll now be the proud owner of Succubus Stories (Steam Edition). Congratulations, I guess...