Lucky Ra's advice is golden. Make sure your cold chisel is sharp. I start by laying the down bevel against the rectangular slot I've made, starting at an outside corner. I use a 3# short sledge hammer to strike the chisel. Then I go to the opposite corner and do the same. The third strike is with the chisel in the centre of the slot. I continue with this until I have raised a considerable amount of barrel steel up above the barrel's surface. I finish the dovetail with a safe triangular file. When the sight fits the dovetail, I remove it, and file the side angled flats of the barrel smooth, and then file the top of the raised metal flat, but leave a good lump of barrel steel above the flat. This type of sight slot is most useful for tightening up a sight slot if the sight becomes a little loose.
I'd post a picture, but our site isn't allowing this now for some reason.