[phi-dan]

Very straightforward, once you've got the cable off!
The cable connection on mine was so corroded I had to take a hacksaw to it, being careful not to cut through the ring connector, of course.
There's several pages on it in the Haynes manual, but basically there's two bolts holding the motor together. Take these out, then separate from the gear end. Make sure to note where the washers come from as there several on each end of the motor shaft, and some are insulating.
Fitting the repair kit (basically a new set of carbon brushes and cable mounting bolt) is just pop them in where the old ones come out of, then put it all back together.
Like you I was worried I might wreck it but, as with most things, if you work through it carefully and don't try cutting corners you should be okay.
Edit: also make note of the alignment marks when you separate the motor so it goes back together the same way.