After knitting a garment with the slit for the pocket opening and completing the knit lining of the pocket, place the lining of the pocket on the inside of the sweater.
Mark the length that the pocket lining is supposed to be. For example, in the photo the pocket lining was 5" deep.
Thread some scrap yarn onto a tapestry needle.
Start directly below the pocket ribbing and choose either smiles or frowns (sometimes called u’s and umbrellas).
These are the parts that make up a stitch on the purl side, if you study the fabric you can see that there are u-shapes (smiles) and upside-down u-shapes (frowns). If you choose one or the other and stick to them, it will help keep your work in a straight line all the way down.
Thread your scrap yarn through every other smile or frown, in a straight line down from your beginning point at the ribbing.
In the photo you can see the scrap yarn is threaded through smiles from the bottom of the ribbing to the 5" mark.
Repeat for the other side
Thread your working yarn (use tails wherever possible) onto your tapestry needle and stitch through every other row on the edge of your pocket lining. Then thread it through the stitch that your scrap yarn is running through on the sweater. This gives you a precise road map of where to go.
Once you have worked down each side edge it should be easy to whip stitch across the bottom
Every other stitch for fine gauge
Every stitch for bulky yarns
I use whipstitch to attach pocket linings. I whipstitch woven fabric linings with a sewing needle and thread (as shown). I whipstitch knitted linings with a tapestry needle and yarn.
Pass needle up through both pieces.
Bring needle to front over the top.
Repeat these two steps for length of seam. If working with thread, knot as end and snip. If working with yarn, work in end and snip.