There are several DIY skimmer plans available on the internet. It just takes some searching. I know of a good number of people who have successfully built their own skimmers at a fraction of the cost of purchasing an equivalent.
the basics are as follows
the water is mixed with tiny bubbles in the skimmer chamber.
Dissolved organic compounds have electrically charged polar regions which are attracted to the water/air interface (the dissolved organics in the water stick to the surface of the bubbles)
The dissolved organic laden bubbles collect in a foam of frothy nastiness at the top of the skimmer and are pushed into the collection cup where they build up until you empty the collection cup. (the stuff you are dumping is referred to as "Skimmate")
The clean water exits through the bottom of the skimmer
the efficency of a skimmer is directly effected by both the bubble size (should be about the size of the ball tip of a ballpoint pen) and the length of time the water remains in contact with the air bubbles.
additional reading: http://www.reefkeeping.com/issues/20...ture/index.php