I've used red leather from a thrift store bought skirt that didn't fit. So far pieces of this skirt have been made into five accessories with more to come. Watch this space for many more 'red leather tutorials'.
However, keeping with the task at hand, leather is a nice stiff fabric that will hold your phone well. I'd recommend this or any other thick fabric that you want to try out.
Fold the fabric in half and mark a seam allowance of about two centimetres (or whatever is appropriate to the size of your phone.) Cut around the fabric with a pair of high quality fabric scissors. Crap scissors will give you crap edges to sew together, and no one wants that.
I bought these studs cheap on Ebay. They're such a rip off in haberdasheries! Lay the studs out on the inside of your fabric. Make sure you leave enough space at each side for the sewing machine foot. I wanted mine to be very neat so I measured the distance with a ruler. Mark where the stud pins will go with a fine pen. Push a thick needle through the markings to give the stud pins a guide to go through. Push in the studs and fold the pins down to secure.
Place the front and back sections of your phone cover together with the insides, out. Sew around the edge leaving a centimetre seam allowance.
I used the hem of my skirt when cutting out my phone case pieces so didn't need to sew it at the top. If you've cut an open piece of fabric then make sure to fold it down at the top and sew a line about two centimetres down to create your own hem. Do this after cutting out your pieces, but before adding your studs.