How can I fit a replacement petrol filler flap?

I have a Ford Focus 1.6 Sport (2007) and I need to replace the fuel filler flap/door. I have bought the part but don't have any mechanical knowledge. Is there any way of doing this myself without sending it to a garage and if so what tools do I need and could I have some step by step instructions please? Thanks.

Lee Sutton
March 2008