The project manager enables users to create/execute and manage migrations with large numbers of Users, Team drives and folders.
Once you project is configured, you need to add mappings. This can be done in a number of ways, the easiest being just selecting mappings from dropdown.
Adding a mapping using the drop down menu
From the project, click "+Add User/Drive". You can then select the source and destination by checking the checkbox's and clicking "Add Item Mapping"
Note: Checking "Don't preserve source root folder" will result in the content from the source going directly into the folder selected on the destination