I'm assuming you have a MK4 Golf.
It has been said on here that Hucho's book warns against doing angled belly pans or Kammbacks on estates because of vortex formation. Saloons and coupes, due to their sloping rear rooflines, create pressure differences on the top and sides and when the air meets at the rear corners of the roof it creates a spinning vortex of air. This is bad because it increases your drag.
Estates, though, are already approximately an equilateral box. There is little pressure difference from the top to sides so you will have much fewer/smaller vortexes coming off.
You have A and you want F. A badly executed kammback could give you Ba
If you're going to extend the rear more than what Piwoslaw has, I recommend you try a partial boat tail. This is because adding too long of a kammback might create a pressure difference! But this should only happen if you by far exceed the vertical plane of the lower hatch.
3-Wheeler has a very nice looking boat tail on his Honda Insight and if you're worried about looking professional that would be the car to copy. Georgboy has a nice partial kammback on his Geo Metro which you may also want to take a look at.
I hope that all made sense. Then again you probably won't be going full steam ahead on the aeromods right from the get-go. Look into a belly pan, which may deliver something a couple mpg highway. For stop and go traffic though you'd probably be better off reducing weight. Remove the rear seat if possible, take out the spare tire if you have a roadside assistance program, and maybe buy some light alloy wheels.
Good luck!