First select the type of stock, the material and the milling unit.
In the following view you can now access your explorer using the "Add" button and select the desired STL file.
With prefabs you also where to position your stock within the holder.
After loading the STL file, select a suitable stock in the lower menu and start the configuration using the "Prepare" button.
Alternatively, you can also import your STL files during preparation. You will find the option for this in the first stage "Import".