For a truly smooth paint job on your figure you can't beat using an airbrush.
What is an airbrush? An airbrush is a small hand held paint gun. The most common you will see for this type of hobby work is a single action gun. See pic.
The bottle of paint attached to the needle assembly and the air inlet attached to the handle. Its a simple process to adjust the spray pattern. The cone over the needle threads up and down exposing differing amounts of the creating anything from a fine pencil line to a 2" spray pattern. The better airbrushes have better patterns and tighter tolerances to help you get the desired line width.
Anyway that’s an airbrush.
What you use is a good paint reduced to proper viscosity so it flows. I personally use testors model masters paint that I find at my local hobby lobby. They run around $2.50 a jar. Always use acrylic paints on plastics such as Star Wars figures.
How to use your brush.
Once you have your airbrush and compressor set up, paints in order and cleaning supplies ready your set to begin. Get your self a few sheets of cardboard to practice on and get started. The first time you use the brush don't expect to be perfect like all things this takes practice. Play with the adjustment settings and observe the rate at which the paint is applied. The idea is to lay down a small amount of paint at a time to avoid over wetting, and runs. repeated light coats is the best. Always seal the paint with clear coat to avoid chipping.
There are a few ways to use your brush to get the results you want.
First you can free hand. This is not recommended for beginners or those with an unsteady hand.
Secondly you can cut a guide from cardboard to use as an edge guide.
Third you can mask off the areas
Fourth. A way I use alot personally is to use something called frisket film. You can get this at a craft store. Its a clear sheet of film that you cut as a template and stick onto your project. Since it has a light amount of glue it removes easily.
Once you get the hang of it you will have projects that will look like the factory painted them. No more brush marks or uneven paint.