Click on the item to bring up the workorder popup screen

Click on cost details

Here you can add new costs/components or edit existing. The components can be stock items, services, raw material, parts or any type of cost.

To learn about how to add a new component, follow the steps shown in https://support.onswim.com/knowledgebase/73/04-How-to-add-a-component-to-a-Stock-Item.html