The support rails are screwed to the inner rear wings. There are holes there to accept blind captive nuts which you slot in, and they expand when you screw the bolts in. There is a parts list for the job on this site.
I cut slots in the fabric side panels. You have to be quite accurate, but there is a plastic trim piece which goes on top to "hide the join".
Getting all the rear trim out is exciting, as you need to be very careful that you don't break some of the attachment clips.
I didn't make a shelf, but I bought a second hand one from eBay.
Took an afternoon to do. Very pleased with the result, as it covers up my spare wheel nicely.
Alan