Airbrushes vary in specification, so it's useful to do some test spraying on spare plastic or cardboard to get the right application before spraying your model.
Humbrol paint needs to be thinned for airbrushing: 1 - 2 parts paint to 1 part Enamel/Acrylic Thinners is usual. Remember, the thinner the paint the more coats you will have to apply.
Keep the paint in the pot uniform at all times when spraying by constantly stirring. This will ensure a consistent finish.
Always spray parallel to the surface being sprayed and don't spray too close as the paint will become heavy and start to run and sag.
Always clean your Airbrush immediately after use to prevent clogging with Humbrol Enamel Thinners, Humbrol Acrylic Thinners or water depending on whether you have used Enamel or Acrylics Paints.
Enamel Paint = Humbrol Enamel Thinners.
Acrylic Paint = Humbrol Acrylic Thinners or water.