Download the .dem file
Place the file in Steam/SteamApps/common/Team Fortress 2/tf
In game, type "playdemo DEMO" (replacing demo with the file name) in console
Also, you can press SHIFT + F2 to open the demoui
Watch this report not get handled because people are too lazy to handle reports on the 3 dead servers (Surf, Smash Bros, & TF2Ware). Like how I make a simple mic spam report on Surf & it gets locked for being old.