Subcollection feature is set up using Metafields and the dedicated Subcollections section. So, the first thing we need to do is create new collections metafield.

To do that head to “Settings->Custom data”. In the metafield section select “Collections” and in the new window click “Add definition”.

You can name the metafield “Subcollections” or something else, but it is important that the namespace and key field is set to: “custom.subcollections”. 

In “Select type” select “Collection” and then select “List of Collections”. Check the “Storefronts” checkbox in the sidebar and save the definition.

Once you have your definition set go to “Products->Collections” select one of your collections and scroll down to the “Metafields” section. There you should now see “Subcollections” metafield you created. 

By simply clicking on it you’ll be able to select collections you want to add as a subcollection for that collection. You can do that for all collections that you want to have a subcollections displayed on their collection pages.

The last step is to go to your Theme Editor and from the template drop-down select the “Collections” and then the “Default Collection” template. Now you can add the “Subcollection” section and set it up the way you want. It will now automatically display subscollections on every collection page that has subcollections set in the metafield.