Use the built in steam backup functionality.
Steam's own backup function is teh suck. Seriously. It's super bad. If you have a LOT of games to backup you have to click a LOT of little checkboxes, one for each game; there's no "select all" button, filtering function, search box and so on. Also, list is not alphabetically sorted, so just a jumble of games in whichever order. And the listview is a tiny little window that doesn't fit very many games at a time, causing a lot of squinting and scrolling if you have a high-res display due to Valve's love of tiny fonts in their GUI...
Second, backup may hang indefinitely at any point during the process (known, years-old bug which Valve, despite swimming in money has never bothered to fix). It is also super slow, possibly because it doesn't support multiple cores/threads; it maxes out one core and bottlenecks in a serious way, making many hundred gigabyte-sized backups take humongously long time to complete, assuming it actually ever does complete.
Then, on top of everything else, if you need to restore just one game from a large backup, you'll find every single game is highlighted, and you need to manually unselect every single one you don't want restored (again, list is pretty much randomized, so you need to scroll and read that tiny listview carefully.)
Or this is the way it worked last I tried to use it around the time when I was about to install Win8 for the first time on my new rig. Maybe they have finally fixed their shit, I dunno, but considering how long backup has been the way I just described I somehow doubt it. Heh.
I'd recommend
@Lightman's method just for the ease of it (it's fast, and tends to just plain work), but then you don't get any uninstall option in the control panel for your games, or any shortcuts inserted into your start screen/menu, if that's the kind of thing you like... Bit crap, really, but what can you do?