Firstly take a sharp knife and make a shallow cut on its belly. The cut should run from the gills down to the anus. With your fingers pull the cut open and start to pull at the organs. The fish insides can be quite delicate, so be gentle! At each end of the fish you should pull at the organs to separate them from the fish's body. Inside the fish's body there may be a dark vein running along its spine which you should scrap out.
Now at this point it is my personal taste to cut the head of the fish off. It can take quite some effort to do so. You can also cut the fins off. When finishes clean the fish thoroughly with water.