The powders need to go on after varnishing, or they will dissolve. You can buy fixatives to stick them down, but I'm told that white spirit does the same thing. You can either mix the fluid with the powder and then smear it on, or paint the liquid on and brush the powder over the top (it will stick to the liquid).
Personally I just brush the powder on and leave it without fixing in place, since I can always put more on if necessary. Put the powder on in different layers, mix it together, do whatever you want, I don't think you can be too precious about it, just slap it all over with a big soft brush and blow away the excess.