When editing a product, click the OPTIONS/SIZES tab in the small tabs area.
Depending on whether your product has Product Variations, you will either see the link option at the top of the page or in the sidebar.
You can use the find products to link search field to return a list of products to add.
You can choose the show related products for only this product, or have the linked products also show the link back to this product.
There are three ways to display your products.
By default related products/associated items appear below the main product information.
If the product makes use of tabbed information, the related products will be shown in another tab.