Products can only be mapped to Subscription Groups so if a Subscription Group has 4 selling plans then all the products would get mapped to these selling plans. Please check how to create selling plan here if not done already. Link here 

Once you have created Selling Groups & Plans, go to "Selling Plans" tab in the Loop app and click on "Add products"

Click on "Select Products". Then select the products you want to map and click on "Add"

That's it :)

Steps to setup subscription only products

To remove the "One Time Purchase" option from your products, meaning the product can only be bought only on a subscription. For ex. 3 month memberships where customer is billed once a month.

To activate products only for Subscriptions - go to "Products" on your Shopify Admin, find the product you want to set as subscription only and click on it. Scroll down to section "Subscriptions" and tick the box "Only allow purchase of this product as a subscription"