To successfully integrate ShipStation with Printful, you'll need to first generate your API Key and Secret.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Account and then API Settings from the sidebar on the left.
Click the Generate New API Keys button.
Your API Key and Secret will now appear.
Once you've copied these down, head to your Printful account and head to the Stores tab.
Now find the Connect to an ecommerce platform box and click Connect.
On the page that appears, enter your API Key and Secret and click Connect.
After you click Connect, select the store whose orders will be importing. Click Connect and the store will be connected.
Repeat this process for each store whose orders will import into ShipStation.