This is a really simple project, perfect for kids or beginner sewers. I've made a pattern which is available here.
I can't put together a step-by-step photo tutorial for you because I didn't think of it till we'd already finished making it ^_^ but I did take a few photos to show how it goes together, and I think it should be easy enough to follow.
All you will need is a pretty button and some scraps of fleece (we used scraps left over from the last batch of Blankie Babies I made). You will also need the usual sewing supplies of scissors, pins, a needle, thread, and a sewing machine.
Using the pattern pieces provided, cut out the back and flap piece from one colour of fleece.
Cut the front from another colour of fleece. We decided to use two colours of thread, so that the stitching would contrast with the fleece - pink with the purple and purple with the pink.
Pin the front piece to the back and flap piece with the wrong sides together and the curved edges even. Stitch together as shown. Use a pin to mark the position of the button hole and button, then put the cellphone inside to check the fit (Mr K's fitted fine, as did Miss Missy's of course, but I would have moved the button a bit if I'd made one for my phone). Move the markings if needed. Make a buttonhole in the flap, and stitch button to the front.
A lovely cellphone case!
Blankie Babies are available for sale at my Felt shop