I only did it the once, and am far from knowing all about flying or the competion, so take what I add here with your own pinch or truckload of salt.
You will launch off into the sky with a bit of paper telling you to go to x - which you will have to i) determine where it is, ii) calculate a heading & ete, iii) fly it, iv) take a photo of it, and then v) open the next envelope with the subsequent point, and then do the process all over again.
The marking scheme is confidential, but involves (from memory) crew experience, nav equipment installed, accuracy of calculations, accuracy of flying.
Bear in mind the markers know your a/c speed, and thus what you *should* have calculated for each leg, its no good adjusting your speed to fit your calculations!
Another tip I would add to the previous, a crib sheet of all the local LARS/Airspace frequencies - you'll have enough to do without having to search for these en-route.
When I did it, we went from WW as far south as Bembridge, as far west as Devizes, with about another half dozen points scattered between. The whole flight in an Arrow lasted us 2 hours
Have fun - maybe see you there