Click on the pencil icon for the sale item you want to add a recipe to.
This will take you to a section where you can add ingredients or choose another item and copy its recipe
The ingredient will now show in the list. Choose a Unit from the drop-down box then choose an Amount which is a multiplication of that unit. The options for the unit are determined by what was chosen in the Product when it was set up. Once a product is set you cannot change the unit type. If you made a mistake and want to use Quantity instead if Liquid volume you would need to delete the product and create a new one.