Ok, this is not exactly a generic pocket; I'm writing this up for ladies who are helping me sew Bible costumes; we're adding inseam pockets so that in the event that any of these folks might need a wireless mike at some point in the future, they'll have a pocket to stick the transmitter in... ;)
However, once you realize the placement marks on these pockets can be adapted for whatever other thing the pocket should be added to, then it will apply.
I blatantly copied a Kwik-Sew inseam pocket, which had a mark for the waist. The sideseam notch on the Bible Costume (McCall's 2339) appears to be at about waist level...close enough for costuming, anyway. So, I put a clip at the waistline mark on the pocket to match to the sideseam notch on the garment. There are three clips on the garment...the armsceye notch, the dot where the dropped shoulder seam turns into the side seam, and the side seam notch. The three notches on the pocket are the waistline notch, and the two notches that mark the pocket opening.
On the wrong side of the pocket, draw a line 7/8" from the raw edge between the pocket opening notches. Match the waistline notch on the pocket to the side seam notch on the garment front, then stitch from the raw edge at one opening mark, over to the line you just drew, pivot and stitch down to the level of the second opening mark, pivot again and go straight off the fabric. Go back and, using very small stitches, reinforce the corners by stitching immediately over them for 1/4" or so on either side. Clip into the corners.
Turn the pocket to the inside, pulling the corners out and press it well. Edgestitch it.
Now, lay the second pocket piece over the first, right sides together, and, moving the garment out of the way, stitch around the outer edge of the pocket. If you don't have a serger, straight stitch it first, then go back and zig-zag the edges. Baste the pocket to the garment front within the side seam allowance.
Now you can treat the whole front/pocket assembly as a regular front; you don't have to stop and start at the pocket opening, and there's no danger of the pocket sewn to the front not matching the pocket sewn to the back. (That photo looks like the pocket opening is huge...it really isn't; I think the telephoto distorted it some).
You just have to be careful not to catch the topstitched edge of the pocket opening in the sideseam when you sew the sideseam...but it looks very nice! :)
This can be used on any pattern that uses an inseam pocket. However, unless your fabric is very firm, for a real life wardrobe, fuse lightweight interfacing to the pocket opening area of both the pocket facing and the garment...that helps it keep its shape.