I will try to explain as clearly as I can how I put the pockets into my McCall's 2016 shirt dress. I've included some quick illustrations I worked up in Paint. They are meant to be only rough approximations of the pattern shapes.
Adult pockets are pretty much one-size-fits-all. The easiest method of getting a pocket pattern piece for your pattern that does not have one is to take one from another pattern that incorporates side seam pockets. The pockets I added to one of my husband's jackets came from a woman's robe pattern. The pockets I put onto the shirt dress came from another dress.
The pattern piece should be shaped something like this :
If it looks like the piece below, with the flattened top portion, it is meant to go into a garment with a waistband. I think you could just trace it and eliminate the top flattened portion (as the blue dotted line illustrates) with no harm done.
To determine the correct placement of the pockets in my dress, I guestimated a little. I measured from the approximate point the underarm seam was going to hit on my body to where the a pocket would be comfortable for me. Then I folded in the bust dart in on the pattern piece and measured down the same distance, making a new notch to mark the pocket placement. To make a matching notch on the back pattern piece, I measured from the notch that the pattern company put under the armscye (for matching front and back when sewing the side seam) to the new notch I'd made, and made the new notch on the back pattern piece the same distance below the existing notch on the back pattern piece.
Cut FOUR pocket pieces. Pocket pieces are sewn to dress pieces BEFORE the side/underarm seams have been sewn.
Right sides together, match the top point of the pocket pieces with the notches you made for pocket placement. Sew at the seam line (represented by blue dots). You should end up with fabric pieces that have the general shape shown in the illustration below. Press the seams flat.
When it is time to sew the side seams (which in the case of the McCall's 2016 dress is after the side seams of the raglan sleeves have been sewn), put right sides together, matching notches and seams as usual, and matching pockets pieces, and sew the side seam AROUND the pockets, illustrated by blue dots below. Backstitch for reinforcement at the top and bottom where you pivot to go around the pockets.
Turn your dress inside out and press. You have pockets!
If you have a copy of the Vogue Sewing book, it also has instructions (a little more fiddly then mine), and a pocket pattern piece drawn on a grid that can be enlarged.