Here is one design I particularly like. It was also one of my first attempts at knitting a piece with 2 different colours in.
I thought I'd try sharing the pattern with you, hopefully it's clear enough (bear with me it's my first pattern and to make matters worse I think I'm coming down with a cold).
If you want to make a phone cosy it's pretty easy. You work a rectangle, fold it in half and sew up the sides and bottom. You are going to start at the bottom of the piece and work upwards.
Firstly cast on 26 stitches with 4mm needles (this is the pattern I used for my Samsung phone - you may need to alter it slightly to get it right for other phones).
Work 9 rows; knit one row, purl the next row, then using the chart below work in the following colours (continuing to knit one row then purl the next).
You will need to repeat this chart once to get the full 26 stitches, or, if like me you prefer to be a bit awkward do the chart once and then the next 13 rows in plain black stitches.
If you need to add any more rows adding them in black after the skull will probably give it a more balanced look.
Knit 4 rows in lack then change to white, knit 4 rows in white yarn. Then change the yarn back to black and change to smaller needles and do 6 rows in k1 p1 rib. You can then finish it off with a crocheted chain of about 30 chains using a 4mm crochet hook.