Don't see how you can go wrong, spray QD on, rub it with bar. Took absolutely ages to get rid of the tiny little black specks on it, had to rub the thing over like a 5" sq area about a hundred times to get it totally smooth, rinse and repeat for the whole car.
Wish I'd had more patience and bought a more aggressive Bilt Hamber one online but I wanted it right away and figured the kit QD/MF cloth/wax would be handy. The MF is nice, QD isn't as good as AutoGlym imo and the clay seems to have a very weak action. I don't really care about light marring as I'd machine it right after anyway, much rather have a bar that removes contaminants in 5 strokes rather than 100.
Firstly, with you complaining that your car went back to exactly how it was - this is not the fault of the clay, but rather your fault for failing to protect your car properly afterwards. The clay is there to remove contaminants, not protect against them.
Clay is not a tar remover, either. Sure, it can remove tar, but you'll have a much easier life if you use a decent tar remover and/or IronX before claying.