Keep in mind that I really don't know what I'm doing. I just found something that works for me...
I use the pastels almost like a wash. If I'm using chalk pastels, I grind a small amount of the chalk into an painters pallet. I fill another well of the pallet with turpenoid and slowly mix in some of the pigment powder. The consistency will vary. If I'm doing something like accenting panel lines I'll use a thick, mudlike consistency. I'm I'm doing just a wash, I'll use a much thinner consistency.
After applying the mix to the model, I let it dry for a few minutes to an hour or so. Then I remove the excess with a clean cloth moistened with turpenoid.