Assuming it's a 6-sided pocket (lower horiz edge, two short vert sides, two angled entries and the horiz top) you'd want to graft the bottom edge, then do mattress on the vert sides, and then finish with grafting along the top edge (make sure NOT to sew the openings!). Helps to take a thinner, contrasting yarn and weave it thru sts on the front of the sweater one st larger than the actual pocket so you can see exactly where to position and seam.
Basic Tech's (above) has info on seaming (grafting for horiz and mattress for the verts). Hopefully you allowed for a selvage st on the vert sides so you'll have a st for the seaming. The upper and lower edges will mean seaming in the CO and BO sts.
ETA: of you've not already done an edge st of some kind (crochet or otherwise) on the angled entries you could incorporate this as you attach the pocket. For example: seam the lower edge, then seam the left vert. Work the decorative edge of left angle. Then attach yarn at lower right and work right vert, work dec edge of right angle, finish by doing the top horiz seam.