I can't remember how I learned to dress a chicken Madchickenlady? It must have been a book or article somewhere and my technique has very slowly improved with practice. One thing I still struggle with is cutting around the vent, because knives seem to be the wrong instrument and it is far too easy to rupture the abdominal sac or slice your fingers. Now I use a Swan Morton surgical scalpel with a curved blade, but I did start with a Stanley knife blade held between my fingers. Whatever you use it needs to be incredibly sharp.
With the neck removed the crop (which should be empty) needs to be separated carefully from the skin. When the vent is cut around and an incision made to the breast bone the abdominal sac is separated from the ribs and skin simply by fingers working around. Once free it is a case of pulling the whole lot out in your hand in one go and the crop and windpipe should draw through with it. Sounds easy, but I still find it difficult.