Use the whole block, you can break it in half if you wish, use one half over the top and tops of doors, then the other half on bumpers and bottoms of doors which are generally much more contaminated. The detailing spray in the megs kit is ok and perfectly fine to use, I however get on much better with some strong soapy water, just keep dunking the clay bar in to keep it very wet. Just what works best with you really.

Best thing I can suggest is you watch this series of videos, about a couple of hours long but crammed full of info and definately worth a watch!