Bind across one end of the quilted piece. This is the end that will be hidden under the flap. I used an odd end of binding left over from an old project.
Fold the cosy over and sew the two side seams. With the mobile phone inside it you can see that there is a generous sized flap to fold over, Also you can see the material I used for the inside. Not particularly attractive, but it will rarely be seen.
Before putting on the binding, trim with the rotary cutter down each side to give a clean straight edge.
Now the first line of stitching for binding is done, with the top two corners mitred. At the bottom left I had a selvedge edge at the beginning of the binding; at the bottom right I had to fold in the raw edge.
Here is the view of the other side...
... and with the flap folded over...
... and of the back.
The elastic can be pulled over to the front when the phone is inside to keep the flap down. The other option is to sew on a button and make a loop to fasten over it. Perhaps I will do that with the second one.