Its actually quite easy to do, you lower the rear by setting the height of the trailing arm & then putting back in the torsion bar - it will only go in one way. Ensure that you hammer the bar back in properly.
To make the bar easier to come out spray the splined ends of the bar with WD-40 or similar a couple of weeks before you plan to lower it & drive the car like this. I re-sprayed the ends of the bar every couple of days & mine came out without any problems.
There is a guide on the site somewhere, but cannot remember where it is. Im sure somebody will post the link or just do a search.
Good luck
James