I doubt it was all 'legal fraud' too. I think there was alot of what they are accusing the GOP doing in NC, which is taking ballots with just a signature and filling them in.
I also imagine there was outright ballot manufacturing, which is damn easy to do if you know who's already voted.
I'm sure there are other tricks they pulled. If you know the process, and the links in the chain, and have the support apparatus in place, you can game the system at anywhere it's weak.