I made a contrast color patch pocket on a sweater for one of my adult daughters last fall. I looked in my Vogue Knitting Book for advice about how to do it and here is what I did. I blocked the pocket, (you may not need to if you are using acrylic, but if it is wool, block it like you will the rest of the sweater) then I laid the pocket on the sweater in the intended spot. I made a long running stitch along the sides and bottom of the pocket, on the sweater, following a row of stitches on the sides and in a straight line along the bottom to mark where I wanted to sew the pocket on. Then I pinned the pocket down to the sweater inside the lines of stitching. Using the yarn I made the pocket out of I used an overcast stitch to sew in on using the stitches for a guide to keep it straight on the sides and using special care at the top corners so it will be strong at those points.