Double Dark Side! Diesel & 8P
I changed my alternator on Wednesday and there's an A3/S3 manual in the stickies that shows a diagram of how the belt should go back on. It does say that I reusing an old belt you should make sure it goes on the same way as running a used belt in reverse to the way it has been going can snap them. A new belt can go on any way though.
The tensioiner can't really be adjusted. You just lever it back into place once the belt is back on.
Have a look in the stickies of the 8L forum and right near the top there are links to the A3/S3 service manuals, you want the electrical one. In there check the pics and let me know if the belt was on right.