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
Warning ... code can pose problems on large meshes ...
Sorry ... the code works but .. I used it on a large mesh and it crashed Unity. I lost about a day's worth of coding. The problem however is not in code per se but in Unity, or rather both. It will not work on large meshes. I am not completely sure why Unity crashed but I am working on a custom mesh with a lot of vertices, about 350,000. As someone pointed out above there should be some way to limit the updating for large meshes. Also, if the length of the normal in the inspector is set to zero, to hide the normal, then when it is set back to a non zero value the code will not work. This has to do with some weird Unity editor updating behavior. In addition there should be a way to turn the method off and on in an easy manner.