You'll need to have a google session looking for spoilers - you can also fit some Lupo spoilers, although most drop down on the outer corners so go over the rear windscreen.
The better quality ones (mine for example, which costs a quite frankly scary £160+P&P+paint+fitting when new!) have little magnets built in to
help with positioning. Then its glued on with some kind of sticky - I had my local bodyshop do mine so I don't know exactly how its done, but it is the easiest way to do it, and means you can't have any accidents