After installing Outlink follow the steps below to add external / affiliate links to any product on your site:

1.  Login to Shopify (Ignore, if done).

2. Save the products.

3. Select the product and go to product details page.
4. (Important) Select a product and go to the product edit page where you can edit the product details (name, description, price etc). Here you will see 'Outlink' button under 'More Actions...'.

5. 'Edit Outlinks' screen is displayed. Enter the external link URL, enter label for external link button, toggle activate 'On' and click 'Save'.

6. External link is added to the product. To verify, visit product page on Shopify. 'Add to Cart' button will be replaced with external link button. Clicking on this button will redirect the user to external site in a new tab.