First go to Settings  |  Company. To enable production for your existing account there is a checkbox located beneath the other opt-in features in Unleashed, multi-warehouse and negative stock.

Enabling production cannot be reversed, and is a permanent decision for your account. Please feel free to speak to an Unleashed Partner before activating this feature.

Before you can start assembling goods from components, you must nominate the appropriate stock codes to be component goods or assembled goods. This is done from the product details page for each of your stock items. You can access this from Inventory  |  Products  |  The specific product.

 You then can select whether this product is a component, an assembled product, or both. If you have products you both assemble, and use as a component in the production of another good, you would select both.

When you have either created or nominated the products you will be using in production you can now create a Bill of Materials.
 

Please note that it is possible to nominate a product as both a component and assembled product. This allows you to use an assembled good as a component in creating further products. Be aware that the value of this product will be affected by the value of the components used to create it.