Ive just built one out of astro, in Crewe Cheshire.
one of the biggest things is make sure you prepare the ground properly first. Put decent land drainage in and level the land properly etc and make sure you dont have any bumps in the surface.
When you lay the astro you will need machinery to do this because of the sheer weight of it. 1 roll of astro weighs over 600kg
once you have laid it all it is then prudent to go around again taking out any lumps and bumps that may have appeared, (there will be many) I personally used a trench rammer for the high spots and used sand to fill the low spots. Do not aim for a perfectly level surface remember it is off road but make sure you take out any bumps that kick you cars about.
my track build etc is on my facebook group
http://www.facebook.com/home.php?sk=...589495917&ap=1
ask to join and ill add you, theres a plethera of photos and videos on there
my website is
www.corc-online.co.uk and will have the build process uploaded there too in the next few days