1.. Get a torrent client (I use BitTorrent personally, but there are several that are reportedly very good).
2. Got to the website.
3. Hit that magnet link, it will open in the torrent client (in BitTorrent, this will pop up a second window where you can disable downloading what you don't want to download, I recommend ignoring this and just hitting ok), hitting ok on this second window will start the download.
4. Get an extractor (I use WinRAR, but most people recommend 7zip).
5. When the download finishes, extract it using the right click menu, there should be options to extract using the extractor program.
6. Play with the exe with the game name.