First: you need a torrent client that supports magnet links.
Second: either you...
- copy the entire link (including "magnet:") and paste it into your browser's address bar (and navigate to it), then click on "Allow" or similar when asked to open the link in a different app. Your torrent client must have registered magnet links on your OS, or your OS won't know which application can handle magnet links.
- or you find the "Add torrent" button in your torrent client, and paste the magnet link where appropriate.