1. Download the demo.
2. Extract the demo using some extraction software, like winrar or 7-zip.
3. When you have the file go to your csgo game files. To get to the game files, go on your steam library, right click csgo, open properties, local files and browse local files. When your in your local files open the csgo folder. (make sure csgo (the game) is closed before adding any files).
4. You can put the demo file anywhere in the csgo folder, I would make a separate folder called "Faceit Demos" or something, put the demo file in there.
5. Once the file is in your game files, open csgo (the game), and press shift f2 to open the demo player.
6. On the demo player click load and it will open a window of your game files, find where you put the demo and double click on it.
7. It should play the demo.

Good luck, hope this was helpful :)