I expect the S80 tensioner is similar to the unit on the 5 cylinder engine. I used a 14 mm wrench to pull clockwise on the "nut".

First, get the coolant and power steering reservoirs up out of the way. They simply slide off a "hook" on the strut tower. You may need to remove, and then clamp the small coolant hose (due to siphoning). This done, you will actually have room to work.

Get the belt back on is harder. The official instructions say to put the belt on the tensioner last. I ended up slipping the belt on the P/S pump pulley last, while pulling on a short piece of rope tied to my 14 mm wrench. You need all of the travel you can get out of that tensioner. In fact, this may well prove to be the hardest part of the job.