Details are on the log book about what to do...or V5 registration document as its now called...
They fill in there details then you both sign at the bottom, they keep part and you then send it off...
Id put it on all places at once Ive had the most success with ebay, just as buy it now, no bids and then usually the prospective buyer will come and have a look and buy it there and then, and then you dont have to pay the final fee as you just remove the ad, so you will only pay insertion fee.
Autotrader next best, then pistonheads last I'd say, Ive never had any interest at all from there. Theres also gumtree, its free, so worth sticking it on. Get your car about thats the best way to sell it