I did mine 3 or 4 years ago, Picked up a piece of astro turf from ebay that had been used just once for a red bull photo shoot so was basically brand new, I also picked up some astro turf foam underlay again from ebay that came from a football club who were refurbing their pitch. A little bit of travelling around but saved hundreds as opposed to buying new.
I dug out the old turf and levelled it and compacted it as much as possible with a roller. I Then got a local paving and driveway company to supply and lay down 3" of type 1 hardcore and compact that with their compactor (my back was not up for that). I then laid a 1" layer of sharp sand down, levelled and rolled it flat then laid the foam underlay and the astro on top. The Astro I had reccomended brushing sand in on top but I never bothered and it stays flat.
Brush the crap up a couple of times a year and happy days, I don't think I would ever go back, the only down side is it can get quite hot in the sun but a quick squirt with the hose cools it down.
my mate is into carpet and he used to fit the stuff I helps him out many a time NO MOW was the stuff any way it is great stuff, as Barlidge has said dig out to level type 1 whacker plate down to compact sand level and roller or whacker to compact then underlay if u wish or just the fake grass, then we used a scacrificer to split the ends for a proper authentic look then brushed rubber granules into the pile to make it stand up, it looks the nuts and I would have it in my garden mantanance free and his has been down for 10 years has moss growing in places but still looks the nuts
If it's in a front garden then make sure it's pinned down well. A mates daughter has woken up a couple of times in the night and looked out to see half hers rolled up ready to be half-inched - seemingly they think it's a good idea until they get about half of it rolled up and realise how heavy it's going to be. I believe they've taken steps now to make sure it's going to be difficult to pull it up.