Mens Short’s back pocket seems very complicated, so it needs a lot of care and minute details to be noted down.
First we will cover just the attachments part. The left one is complicated, the right one is simple.
Make as many notes as possible on the left pattern paper. Here is a closeup. The side seams and fold measurements are noted down. The position of the slit(distance from the bottom, left and right edges), the flap measurements are the most important.
Cut fabric and interfacing based on the above pattern. It needs 4 pieces, 2 of fabric and 2 of interfacing. Press the interfacing on the fabric pieces and mark the slit and flap and seams on the interfacing and you are done!
DO NOT CUT any slits now. Just draw the slits on the interfacing after attaching it to the fabric. We will see about the slits later.
**Now cut a thin cotton fabric as per below back pocket pattern. MARK the slit area, DO NOT cut it. Notice that the slit is a bit curved. Give the same curve on back leg of the shorts fabric. I noticed the curve on the old shorts back leg, curved towards the top not towards the bottom. So not sure as of now which way the slit should curve.
One more interfacing should be attached to the back leg. **
Watch this to get an idea, although the pocket I have is different from what is shown in the video.