You can create a Bill of Materials from the Manufacturing > Bill of Materials page. From the Create New drop-down menu, select between For a New Variant or an Existing Variant. For a New Variant option, you'll be prompted to create a new Manufactured Variant and then the BOM. Within the new BOM, enter the finished Variant name, Yield Quantity, Components, Other Costs (can be added later within the Production Order), and Notes. Click on Create to finish.
You can find more information on the Bill of Materials here.