Step 1: Mask Your Minis.
Masking off the rest of the model is really essential if you are new to an airbrush, but if you feel confident and you have a steady hand, you can probably pull this off without it.
I decided masking was a good idea. I did so by stuffing the models, mounted to an old paint pot, into a plastic bag. I then nicked a little hole with scissors and bushed the business end of the meltagun through the hole. Then I twisted up the bag and clipped it in place. as you can see here.
Step 2: Prep your colors. I chose my colors, shown below and pulled out some mini paint pots I got from an art store.
Instead of buying a whole new set of airbrush paints, you can mix matt medium and water to any acrylic and make the appropriate mixture so that the paint flows well through the brush. You need a special consistency - people say that it should be like milk but that never seemed to help me. It should be runny, but opaque - that is it should not run clear.
Don't get me wrong - airbrush paints are awesome. They skip this step - you can squirt them right into your airbrush...but they are not a must-have, as you can mix up your own.
Here I started by spraying a small amount of vermin brown about half way up the barrel of the meltagun. Then I cleaned that out and sprayed a slightly smaller amount of black onto the end of the meltagun. No real trick to it, just use your eyes.
Step 5: Remove the model from the bag and viola!
Here are some pics of my final product. I had some extra paint so I grabbed a nearby Battlesuit and gave it the carbon scoring as well. Since it was a large model and the gun really stuck out, there was no need to mask it. Enjoy!