I've always liked the look of these, so I tried to make one myself.
First you want to get a neck tie that isn't being used anymore.
Measure where you have to cut the tie, by putting your phone in and checking what is comfortable for you. If necessary, re-sew the middle line of the tie.
Fold the hem inside and pin it. Then sew it, so it looks a little neater.
Now pin down the sides, and sew them. I used a back stitch here. In between steps, keep checking if it's still the right measurements.
Now sew a nice looking button in the middle of the lower part, add some elastic to the flap, and you're done!