Here’s a little trick that will make this job easier and give you a very tidy result.
So you’ve finished knitting a garment with pockets and you are faced with this:
Start by marking the length that the pocket lining is supposed to be, for instance, in the photo below, the pocket lining was 5” deep:
And now 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 you 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 I’ve gone through smiles from the bottom of my ribbing to the 5” mark.
Now repeat for the other side:
Now thread your working yarn (I DO use tails wherever I can) onto your tapestry needle and stitch through every other row on the edge of your pocket lining, then 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. And here is your end result:
Perfectly straight lines and so much easier than guessing what to stitch!