To create a composite variant:
In Inventory, select the product you need to add a composite variant to. Then, select Add a Composite Variant.
Create the Variant as you normally would, then click the green Save button. You will then be taken to the Composition tab.
Click +Add Variant and search for a component. Select a variant to add, then adjust the quantity as needed.
If you'd like to remove a component, click the Trash button on the right-hand side.
When you're done adding the required variants click the Save Changes button.