I must disagree with the previous poster on a couple points.
I use the same generic "PAINT THINNER" from the hardware store you're probably using. Unless I'm really careless and apply too thick a coat, which may whiten somewhat, I've never had a problem, The whitening has happened maybe twice in 25 years and hundreds of applications. It went away with a judicious aplication of Dullcoat tothe affected areas.
I shake the bottle vigorously to mix the carrier and dulling agent. Heck, you'd shake a rattle can, right? Again, in all the years I've used Dullcoat, I've not had a prolem with bubbles when I pour it into the color cup. Any that might persist are broken up when I stir in the thinner.
I use an eye dropper to get the Dullcoat to a watery consistency. I have no idea of the ratio. To some degree, it depends on how fresh your bottle is. As the carrier evapoprates from being opened, you will need more thinner to get it through the airbrush.
Is my method successful? As I said, I've been using this same method for Dullcoat (and Glosscoat, though that needs lees thinner) for some 25 years. It works well enough for me to have a couple hundred awards stashed away in the cellar and to have taken firsts at the local, regional and national IPMS levels as well as golds at AMPS East, and silvers and golds at figure shows.