You'll need two patterns - A and B and felt lining about the same height as the two patterns combined, you'll see why later.

Join pattern A and B together, and make sure that the felt is a bit smaller both length and width wise. The excess patterned fabric will be the seam allowance used for hemming the felt.

Once you've hemmed all the sides, stitch along the line where the two patterned fabrics are joined to ensure that its all bendy and that is the bottom of your phone cozy.

Next, fold up pattern B to meet pattern A (this is where the bendy part comes in handy) and just sew along the sides. Try to be a bit neat cuz you're not turning this inside out or anything; make sure its secure as well.

you could add a button or something to close it with; I used a sew-on clasp and added a cute little bow :D It is advised to add a tiny embellishment cuz it boosts the bags' cuteness like 57% :P
Anyway, you're done with your phone bag!!! YAAAYYY! Be sure to post your version!