On the left-hand navigation, you'll see a + sign in a dotted box. Click that and you'll be taken to the Sales Channels page.
This is where you can select a sales channel to sell on.
To sell on Stitch, click the "Connect to Stitch" button.
You'll be presented with a modal, where you can enter a shop name and your PayPal email address.
Note: If you are already selling on the Zibbet Marketplace or A.C. Moore Marketplace sales channel, then your shop name will be the same, and therefore can't be edited.
Click "Get started" and the Stitch sales channel will be turned on.