The pattern I'm using is from 1982 and the pocket they use is 14 X 16.5cm. It looked huge in the magazine picture. I found a smaller pocket from another pattern and used that.
I started by cutting out a cardboard piece the size of the pocket without the seam allowances as Peter suggests in his blog. I placed the cardboard template on the wrong side of the fabric and ironed the seam allowances over the edges of it. WARNING: be careful what kind of cardboard you use! I used a frozen pizza box and it stuck a bit to the fabric as I was pulling it off. Fortunately none of the writing came off but from now on I'll use something else.
I added the 2cm that was called for the upper edge but then realized that it would be too small if I wanted to fold it over twice. So my first hem was as small as I dared. I made the pocket a bit shorter so I could have a bit bigger hem for the top edge. With wrong sides together I stitched the top edge to the side seam. Then I turned it right side out and top stitched so the flap inside wouldn't flap around.
I liked the shape of this pocket. It was easy to fold around the cardboard template and easy to sew with straight edges and no curves. I'm not sure if it is in the right postition but once my son tries the shirt on he can decide himself where he wants it. And if he wants a pocket on the other side too and the shape of the pocket.