I did mine at the weekend. As it's not even mentioned in the Haynes manual, but is very easy, I took some pictures. Here's how to do it (takes 5-10 mins).
You will need:
1) Awl/small punch
2) Small Hammer
3) Crocodile nosed pliers
4) New hinge bracket (£6.11 inc VAT)
Step-by-step guide
Click on the thumbnails to see larger versions.
Punch out the 4 pins on the existing hinge (look at the new hinge to confirm locations).
Remove the flap & old hinge bracket and clean up the bodywork.
Remove the spring from the flap (top first) and unclip the old hinge (doesn't matter if you break this, but take care not to bend the metal hinge spindle on the flap.
Take new hinge bracket out of packaging.
Carefully press hinge bracket onto flap (again taking care not to bend the metal hinge on the flap) and (inset) replace spring, bottom first.
Position the bracket on the vehicle and carefully tap the top and bottom pins into position. This will hold the flap in place whilst you perform the next (and trickiest) step. Use pliers to hold the last two pins steady while you hammer them in. Once started, it can be easier to use the awl and hammer to tap them fully home.
In place of the last step, you could do as v70_cymru has and use 4x½" No.6 stainless steel self-tapper screws to secure the bracket to the car.
Carefully (this is the bit that broke before, don't want to break the new bracket just yet!) press the spring into the clips on the bracket.
Job done!
I imagine it will be similar for other models.