I made an iPhone cozy with worsted weight, RedHeart super saver yarn. I improvised the whole thing project but you can copy my results with these steps:

Cast on an even number of stitches needed to get 2.5 inches (slightly larger than phone width).
Knit in garter stitch for 11 inches (the phone is 4.5 inches tall, so 9 inches will cover it. The additional 2 inches are for the folded over just before the button loop.
I made up a unique way of creating a loop for the button hole.