It's a=just a standard welt pocket on an angle. Find a welt pocket tutorial you like and follow it, but on the outseam :)

The only trick is that the inseam has to be open when you add the pocket to the outseam.

I haven't made a paper pattern of it, yet, because I've cut it differently each time. When I settle on a design I will make myself a pattern -- but it'll be pretty specific to me.