Here´s how I do it:
You take a cup with a little bit of water, put in one or two drops of black acrylic paint. Then take a brush and put some of the water/acrylic mixture on the object. Leave it for some seconds up to half a minute and then wipe the mixture with a paper towel away. This you´ll have to repeat until you got the effect you want to create. Hope this helps?
I found an older plastic lint roller in one of my greeblies boxes, which has a slightly bigger diameter than the end-tube of the rocket. It also has a perfect hole, where the screw of the rocket can go through. This lint roller will sit inside the jetpack at the end.
Then I cut away the handle. I´ll cut the lint roller tube shorter in the end, so the end-tube of the rocket will slip in till the screw comes out completly.
Now I used the tube draw a guideline, where to cut the hole for the rocket end-tube. After that I started drilling holes, which I make bigger and bigger, so I can use a dremel to finish the big hole.
I have a helmet that I need to fix up and paint, clam shell gauntlets that I need greeblies for, I have a jet pack that I need to paint, I have a chest display, and Wookie braids, and I have the lights for an RF.
I know this so well! I´m working 2,5 years on my Boba Fett now... and finally I see the end... (before upgrades start ) But I´m lucky, my wife (she isn´t a Star Wars fan nor interested in costuming) let´s me do what I like.