One of the easiest ways to knit a pocket onto a cardigan.
The fold up pocket is achieved by knitting the front longer than the back.
Any where from four to six inches--depending on how deep you want the pocket to be.
This technique works best on hip length sweaters.
On some cardigans I've attached a rib trim to the pocket. Knit the rib until it measures, at least, two inches [5.08 cm] and is as wide as the pocket.