Cut off the head and tail. Snip off the fins with scissors. Slit the underside of the fish along the line between the head and tail and remove all the insides. It's a bit messy but easy. You will need a very sharp knife though and remember to descale first otherwise you will never get rid of the scales from your kitchen. De-scale by running under a stong cold tap and gently scrape the fish all over against the scales.