Navigate to a product page and click the 'Related Products' tab.

In the text box, start to type the name, or part of the name of the product you wish to create a link to.

A list of products will appear – select the correct one. Then click 'Add New' and it will appear below. You can repeat the same process to relate more products with this one.

You can delete any relationships from the list too.

Don't forget to click 'Save' to store the changes.