You will want to cut your fabric so that it is twice as wide as your phone (with case if you keep it in a case) plus about 2-3 inches. So my phone is 2.5 inches wide. I cut my fabric about 8-9 inches wide.
I cut the length of my fabric at about 14 inches and then cut off excess before I sewed it. Measure your phone’s length, double that and add a couple of inches.
Now I have a piece of fabric that is about 9 x 14 inches. Cut a piece of fusible fleece the same size and iron it to the back of your fabric. (I will tell you that that is going to add bulk (and cushion) to your project. If your sewing machine struggles with a lot of bulk, use a lighter weight liner).
Fold your fabric in half lengthwise (so that it is 4.5 inches by 14 inches) and stitch across one of the short sides and down the long side.
Turn it all right side out. Press it flat.
Now grab your phone. Fold the wallet up and test how far up you want it to go to match the size of your phone. Cut off any extra length that you don’t want.
Fold under the unsewn edge. (Folding it all inward.)
Stitch this edge closed.
Now, fold the edge you just sewed up as far as you want it to fit your phone and sew up each side. Your phone should now be able to slip inside.
Now to make the little strap, cut a piece of fabric about 4 inches long by 2 inches wide. Fold it in half with right sides together and sew up the long side. Turn it right side out and press it. Then fold it in half, loop your clasp though it and sew it in place in several places on the back of your wallet.