It's meant to be simple and easy to use, and therefore it has been made as a custom editor of MeshFilter. It needs no preparations.
Drop the file in an Editor folder.
- store the normals length i editor prefs
- adjust the normals length in the mesh filter inspector
I just dropped the script in an Editor folder and I cannot see anything. Dropped a Cube in the Scene, select it or not, what should I see? In the inspector I see the "MeshFilter" with the Mesh field, and nothing else. I am on 2020.3