It would've been ideal to spray B12 in the intake while the vehicle was running, it helps break it down and remove the little stuff first. As for the heads, spray some B12 and let it soak a little and whipe it up out with a shop cloth. The heads are easier to clean than the intake runners, which you can spray with B12. I used a baby bottle cleaner to help clean the runners and you will want to cover the entrance with a shop cloth to keep the bristles from flicking dirty B12 fluid on your body and clothes, which will be very hard to remove.
Once you have the intake clean, you can keep up with it by spraying some B12 through the intake every other oil change or whatever you preference is. I do this and build up is not a problem. I use maybe half a bottle of spray with the up keep unless I ignored the cleaning for a little bit, in which I will use a whole bottle.
Edit: If you use anything other than B12, go ahead and buy a box of 12 cans or more because it will not get the job done as quickly as B12 and will not be as clean. And, do not allow these cleaners to come in contact with the outside finish on the intake or valve covers, as it will eat away the finish and have your intake looking like crap!