Cut every piece you need, lining is optional if you do want to make that simply decrease sizes of wallet by 0,25cm and hand stitch it in.
Take the two long strips of fabric. During sewing leave the zipper open until the point where you have to go past the zipper thingy and close it.
Take the cassette tape part and bottom part and place it right sides together. sew till corner of cassette tape (on picture marked with red line)
fold sides of bottom over and sew (if you have trouble folding it or holding it down while sewing snip the corner a bit but not entirely to corner)
Do the same with the other side and you should now have this.
Facing wrong side of wallet sew in zipper, leaving zipper slightly open at all times! Sew other side.
When you have finished the wallet take the cassette.
Open it and remove all things inside; all things includes plastic pointy things which you can remove by use of needle nose pliers
Once the cassette is all smooth glue it onto the wallet (keeping in mind to place right side up).
Wait till one side dries because you want the two parts to be on same level and they may slide once you try to glue them both onat the same time.
When the glue has dried on both sides you can begin to sew in the lining by hand.