Get someones help, and get under their with a torch.. Get them to push the bumper in, and when you see the hole lining up, pop the bolts in

Repeat for the other side... I found mine a bit of a pain the first time, and i can never get them on without someones help! Also, it never made any difference to me for OEM or aftermarket, they were both a pain!
Some people mentin you need to support the engine to do this, even the man in the VW gaRAGE told me this, apparantly one of the Haynes manuals says it too. Mine doesnt say this at all, nor the Specialist VW garage i take my car to (they know FAR more about VW's than VW!). I have never needed to jack the engine up, but just as a thought, you know it has been mentioned by a few people, so you can try and get the jack out if all else fails