I have done a tutorial on star fields with The Gimp before, but I have found a new method that yields much better results. It’s very easy and can be done in a few minutes.
Start up The Gimp and create a new image. In this tutorial, I’ll use a 480×320 image (otherwise it would be too big for this page), but you’ll probably use something bigger. Fill the background with black and create a new layer named “Small stars”. Fill this with black as well and add some RGB noise (Filters -> Noise -> RGB noise). Uncheck “Independent RGB” and set every color to 0.5. Set the mode to screen. This won’t make a difference against a black background, but it will if you want to put other layers behind the stars.

Now we’re gonna adjust the density and brightness of the starfield. Open the brightness and contrast dialog (Colors -> Brightness-Contrast) and set the brightness to -40 and the contrast to 40. This will filter out a lot of the noise and create something that already looks a lot like a decent starfield.

Duplicate the layer with the stars and rename it to “Big stars”. Scale this layer x2 (Layer -> Scale Layer). You will probably end up with a layer of blurry dots. To turn them into stars, open the Treshold dialog box (Colors -> Treshold). By moving the left slider, you can control the density of the second layer of stars and it will sharpen the dots. Here’s the result:

This already looks like a pretty good starfield! For a finishing touch, you can adjust the opacity of the small stars layer. The final result, with the small stars layer’s opacity set to 70%: