Star trail images are created by stacking a large number of individual shots taken from the same vantage point over the period of a few hours. You focus, set up the shot and let the intervalometer do all the work, easy.

Once you have all your images you can load them into free software such as StarTrails or StarStax to create your final trail photo. Sometimes it is fun to generate a video from the images, you can either generate a time-lapse showing the stars circling around the pole star, or as in this example, ask StarStax to save an image after each action which enables you to capture the trail building up, you can then create a video from these images using PhotoShop or Windows Movie Maker.

You can play around with the frame rate etc until you have something you are happy with. Good fun and a great way to make use of all those images!