Usually when knitting a cardigan with pockets you knit or crochet the opening in the sweater and then line with another knitted or crocheted piece on the wrong side.
However if you want to put them on the outside then make your pockets. Line up your pockets with a row of your sweater so it's straight.

I'd sew them on but some of the stitching might show. You can do a decorative embroidery stitch with your wool or a contrasting yarn. You can also crochet a line of single crochet to hide the stitching. This would reinforce the pocket stitching too.

I believe you can still knit inner pockets on the garment, even after it is finished. It's called the "Afterthought Pocket and the directions are in Elizabeth Zimmerman's "Sweater Workshop", and they may be in one of her other books, too.