Model airbrushes are operated by compressed gas, either from a compressor or a can. Paint and thinners on a 50%/50% basis usually achieves the correct consistency for best results.
Parts to be painted should be completely free of dust and grease. Use filler and sandpaper to smooth uneven surfaces. Although not essential, a primer can help ensure that paint adheres and gives a better coverage. Even application with full coverage is the golden rule for painting. Don't overdo the spraying of each coat or runs will occur. Build up the density gradually, allowing parts to dry in a dust free room. Once completely dry, repeat the process to achieve a perfect finish.
For best results, experiment with paint consistency - the nozzle size as recommended by the manufacturer will usually, but not always, be correct. Airbrushes can give fantastic results but the necessary skill must be built up by patient experimentation.