You’ll need a pair of #6 needles and some worsted weight yarn (any material of your choice: cotton, acrylic, etc.)
Cast on 30 stitches.
k1, p1 until length gets long enough to cover phone.
(About 5 to 6 inches will fit a Samsung Galaxy.)
Bind off until you have 5 stitches left on needles.
k5 and repeat for the next 30 rows. (This makes a little handle.)
Sew up seams to form sock. Sew that one end of the little strap down to form a loop. Weave in ends.