Here is how it works:
  1. Install Outlink app
  2. Using the app attach external links to the products you want. 
  3. Add to Cart button on Shopify will be replaced with Outlink button pointing to the external/affiliate website
  4. Your visitors can click on Outlink button and get redirected to the external/affiliate site
  5. You get paid as per your commission rate terms with the affiliate network.

Hope this helps.