David Hodson's answer is half-correct. It *is* just a standard HDD swap, however:
1) A guide to installing Leopard or Snow Leopard won't help because the 2011 Mac Mini won't run any OS X older than Lion. You *could* use another computer to install Lion on the SSD before fitting it, or wait for and buy the USB-stick version of Lion. However…
2) The good news is you should be able to just fit the SSD blank. I believe the Mini's "Internet Recovery" will allow you to connect to Apple and download and install Lion directly. All you need is your WiFi key or Ethernet cable :)