First, I decided what sort of afterthought pocket I wanted to knit, using the advice on the treatment page. The gauge of the existing fabric (60st/10cm) was a big factor. I chose a single layer, knit down pocket, with the pocket bag sewn on to the jumper (rather than picked up as I knitted).
At the same time, I calculated the transition for my pocket - how to change from the very fine gauge of the jumper to something knittable. I knew the gauge of my new fabric, as I was using the same yarn and needles as for the replacement cuff. The new gauge was 30st/10cm, so the calculation was easy: a multiplier of 0.50, and a plain transition of 2tog all along the row.
Next, I marked out the position of my pocket on the jumper, using a basting thread. I marked out the cardigan opening at the same time, to help me visualise the finished effect. I tried on the jumper to check I was happy with the position.
When I was ready to start the pocket, I carefully stitched through each stitch on the rows above and below the row I was going to open. Then, I cut a central stitch and carefully undid the row, stitch by stitch.
Then, I transferred the stitches from the top edge onto a 1mm needle. My first row was the transition, skpo (easier to knit than k2tog) all along the row. Then I carried on in stocking stitch.
When I'd finished the pocket bag, I calculated the edges of my pocket. (If I had been picking up as I knitted, I would have had to do this at the beginning.) The jumper was 82r/10cm, and the new pocket 36r/10cm. This gave a multiplier of 0.44, and I looked this up in the pickup spacing table to find the spacing pattern of 3, 2, 2, 2. I sewed the sides of the pocket very carefully, staying in the same column in between stitches, and joined one row of the pocket to 3 rows, then 2 rows, then 2 rows, then 2 rows (and repeat) of the jumper. At the base of the pocket, I grafted the open stitches to the jumper fabric, joining one pocket stitch to every 2 jumper stitches. It was hard to stay in line - the sewing took longer than the knitting! - but I think it was worth it in the end.
The last stage was to pick up the open stitches from the bottom edge of the opening, and knit a little trim for the top of the pocket. Again, I worked a plain 2tog transition, but this time did a garter version, and then went into double moss stitch. When I'd cast off, I sewed the edges of the trim down and tidied away my ends.
Check out the previous step: stitch-hack initials.
Check out the next step: cardiganise.