The manufacturing BOM is a sophisticated bill of material application, and can be a bit intimidating to use, at first. But here are the steps to create a simple, functional BOM, to give you the feeling of the system; assuming you have your items all set up:
First we want to set up the finished good item so that it is identified as a "Make" or a "Make or Buy" item.
Navigate to: Cards >> Inventory >> Item >> Item Engineering Data, and select "Make" for the "BOM should treat as" value.
Navigate to the Bill of Materials Entry screen: Cards >> Manufacturing >> Bill of Materials
Enter your finished good item.
Click on the Green + to add components. This will activate the component entry area of the screen.