Drafting an open-top pocket is slightly more involved than the side-seam pocket, due to the special facings and shapes involved, but after a couple of tries you’ll get the process down.
Begin by tracing top section of the trouser front pattern on a fresh piece of paper. Add the 1/4″ seam allowances. You really only need the waist seam and a bit of the side seam.
Next, draw the pocket opening marks. These are the most critical part of the pocket, and should be transferred to each piece as you go. I made mine 4 1/2″ from the top of the side seam in each direction, but you could go a little longer or shorter depending on your hand and trouser size.
Draw a construction line between each pocket opening mark, from the seam allowance.
Draw the shape of the pocket, from mark to mark, as desired.
Add a 1/4″ seam allowance to the pocket opening. Adding the seam allowance separately allows you to have more exact control over the position and shape of the pocket.
Draw the front edge of the pocket bag. This should be about 1″ or so from the pocket opening mark on the waist seam, and taper towards the trouser front to help the pocket to drape more naturally.
Draw a construction line at the bottom of the pocket bag, perpendicular to the side seam. I generally make these pockets about 1″ longer than my hands reach.
Draw the pocket curve. I like to deepen the curve near the front so that loose items will better stay in place. I include the seam allowance when drawing these curves, tapering it into the side seam as shown.
Draw the bottom of the pocket facing about 2″ below and parallel to the construction line you drew earlier.
Draw lines completing the pocket facings. At the top, draw a line perpendicular to the waist seam, about 1/4″ – 1/2″ from the front edge of the pocket and at least 1/2″ from the pocket mark. At the bottom of the facing, do the same, about 1 1/2″ below the pocket mark.
Here are all of the pieces traced and cut out for the open top pocket. The pocket bag should be cut on the fold as shown. The rear and front facings can have a seam allowance of 1/4″ to 1/2″ added along the front edges if you have a fabric that frays easily. Be sure to carefully mark the pocket openings on each piece.