How do I add an assembly or kit item?

Assembly and Kit items are types of composite items, so first enable Composite Items under Items Settings. Then go to Items and then + New Composite Item and choose the type:

Assembly item - Items you physically assemble from individual items (stock of the individual items are reduced and the finished composite item gets its own stock).
Kit item - Items bundled and sold together without physical assembly (each item’s stock is still tracked individually). Ideal for gift sets and combos, and you can set custom kit pricing.

Refer to the help document for detailed steps on adding composite items.