To enable the alternative products feature please go to Catalog > Settings > General > Alternative Products and check the Enable alternative products checkbox:
Once the alternative products feature is enabled you will see the alternative products box on the product edit/add screen:
In this box you can specify alternative products by product ID or SKU (comma separated).
Once the alternative products are selected they will show up on the product page as new tab or section:
On the screenshot you can see that this product has 2 alternative products assigned.
We used the Alternative Products Table design to display them but you can use any other listing template to display alternative products:
