The principle glory of the afterthought pocket is that it is, well, an afterthought. You don't have to plan it ahead of time, you can work it after your sweater is completed, you can even put them in several years later. You think to yourself, Gee, I want pockets in this sweater. So whip out 3 dpns, scissors, matching yarn and a yarn needle and do your thing.
The Afterthought Pocket, blocking
To begin working an afterthought pocket, you simply decide where you want your pocket to go. Try the garment on, stand in front of a mirror, and attempt to put your hands into pockets. Where your hands first meet the garment, there should the pockets be. Take off the sweater and mark where you want the pockets to go, and measure out approximately 4-5" worth of sts (about 1-1.5" wider than the width of your hand).
Now, with one of your dpns, pick up the sts in the row above the row you have marked for the pocket opening. In my case, this was 20 sts. Next, with the second dpn, pick up an equal number of sts in the row below where you want your pocket opening to be. You should have a needle holding sts, a row of knitting not held on the needles that lies between the two dpns, and then a second needle holding an equal number of sts.
Pick up your scissors - and taking a deep breath - snip a stitch in that in-between row, right in the middle of the intended pocket opening. With the point of your yarn needle, pick the yarn in the snipped row out of the sts that are held on the needles above and below the opening. It is important that on both ends of your opening you do not take the loose yarn completely out of the last top and bottom held sts, but rather leave the yarn passing through one st once, then once down (or up, as the case may be) through the corresponding stitch at the same end, on the other needle. This helps to prevent a gap at each side of the pocket where the knitting on either side isn't joined to the pocket area. It should look something like this-
Now, using your spare dpn and the sts on the bottom needle, use the Knitted Cast-On method to cast on 3 sts at the right side of the pocket. Work an I-cord BO by *K2, K2tog tbl, slip all three sts back to the left needle*. Repeat until you have bound off all the sts on the bottom needle, you should have 3 sts left on the right needle - put these on a coiless pin to hold for later, and break off the yarn leaving a 6-8" tail to be woven in.
Working the I-Cord BO
Next, work the sts held on the top needle for several rows (about 1-1.5 in.) in the same pattern as your sweater. Switch to stockinette st, and if desired, you can gradually increase a total of about 1" worth of sts at the sides as you knit. At the same time, I knit in the tails from either side of the pocket that are left from the un-picked row that formed the pocket opening. Knit your pocket to the depth desired, and leave the sts on the needle. Break off the yarn leaving a 6-8" tail to be woven in later.
Now, turn your garment inside out, and carefully pin the pocket to the inside of the garment. With matching yarn and yarn needle, sew the pocket into place beginning at the right upper corner, down across the pocket bottom, and then up the left side of the pocket. To sew down the pocket bottom, pass your yarn needle through a purl bump on the sweater fabric, then through a held st on the dpn. As you sew the pocket sides, be careful to leave a bit of slack in the pocket fabric so that from the front side the pocket doesn't pull on the sweater fabric, and remains invisible.
Finally, use your yarn needle to graft the 3 I-cord sts left at the edge of the pocket trim to the garment, and then weave in all ends. When you block the sweater, be sure to run a blocking wire through the center of the I-cord trim to prevent the pocket trim from sagging.
Voila! You have pockets!