1. Create a navigation menu called Cart upsell. The handle needs to be: cart-upsell
2. [Trigger product] Add a menu item and name it following this structure: "products/product-handle". This product will serve as the upsell trigger (the upsold product will be displayed IF this product is in cart).
3. [Upsold Product] Select the product to be displayed as the upsell for the trigger product defined in step 2 above.
4. Enable the upsell in Online Store > Theme > Customize > Cart > Sections tab > Scroll down to the Cart Upsell section.