I've seen it done on that Wheeler Dealers TV show but its always struck me as something that has the potential to go very wrong so i've never dared to have a go myself, I think my fears come from seeing an Astra that my girlfriend owned after the previous owner had a go and the roof looked like a chess board!!

I've watched some good tutorial video's on youtube if you search the subject on there, failing that i'd be tempted to pick up and old bonnet or boot lid from the scrappers and practice on that first.

Edit - i just searched 'mopping vehicle paint' on youtube and there are plenty of video's on there.