Some time ago I bought Knitting Without Tears by Elizabeth Zimmerman. It’s not a book for beginners, but if you already know how to knit, it has good advice, and interesting techniques. One of them is what she calls the “Afterthought Pocket”, in other words, a pocket added to a garment after said garment is finished. This technique is used for those occasions where you finish a sweater, pullover, cardigan, vest, etc. and once you try it on, you wish you had added some pockets.
This technique is for the brave, since you have to cut your fabric! THE HORROR! I have to admit I haven’t tried this technique in an actual garment, but maybe you will. Make a test like mine first!
The following video shows how I made this first attempt. I did it with contrasting colors so the pocket would stand out more. Sorry for the poor quality of video and audio, but I think you’ll be able to see what I’m doing.