This process is pretty simple. Just install the Google Drive for desktop software and move files from your old service to your new one, using File Explorer (Windows) or Finder (Mac).
Unfortunately, the worst service to migrate from is Google's own Google Drive. This is because Google Documents are not downloadable. To get around this:
Note: This won't transfer Google Forms correctly, since there is no file type they can download as. See below instructions for Forms.
Unfortunately (again), Google Forms are not easily moved to GSuite. Below is the easiest way to move them: