To add a new product to Stitch, simply go to the "Products" page and then click on the green "List a new product" button in the top-right corner.
You'll then be presented with a modal where you can select what sales channel(s) you want to list the product on. You can list on as many sales channels as you have connected to, at the same time.
Clicking continue will take you to a page where you can fill out your product attributes.
Once you've filled out the page, you can choose to "Save as draft", or "Publish".
"Save as draft" means that the product is not active / viewable to the public. You can come back to it later to "Publish" it, which will make it active and available for purchase on the sales channels that you've selected.