I have a lot of posts about AutoCAD and a few about grammar, but I expect this will be the only post I ever make that combines the subjects.
Autodesk founder John Walker, a very prominent figure in AutoCAD’s history, has written an article about the correct use of the apostrophe. While I enjoyed it, I think it’s too insulting to be useful. People struggling with the correct use of the apostrophe are unlikely to get past the part where they are called morons, to read the useful advice below.
If you just want simple apostrophe advice without being belittled, I suggest you look at the main page of the The Apostrophe Protection Society instead.