Tack the cup and put it around you phone and see if it fits.
Next hot glue Bobbie pins around your cup leave an cm or two apart until fully covered
Then you tack your yarn and wrap around your Bobbie pins until you go around twice then tack the bottom layer and pull it over with needle
One you get it the size you want close the bottom like you would when you crochet and leave some string out and hot glue a Pom Pom to the end
It should look like this when your done. A fun and easy way to spice up your phone