For this you will need a tube map (grab one for free from any underground station) and the original Oyster card wallet that it came with to make the template, or you can just wing it.
Draw around the wallet on to card.
Cut out the card template.
Trace around the template on to the tube map and on to some paper, which will line your wallet.
You should end up with two pieces that look like this.
Stick the map and paper together with spray adhesive.
Cut two strips of clear vinyl, big enough to cover the wallet and two more strips which are half as wide as the wallet.
Place the map in between the two sheets of vinyl and place the two smaller squares at either side, these will hold the cards.
Sew around the outside.
Cut around the wallet, trimming off any excess vinyl.
Pop your oyster card in to make sure it fits.
And your ready to go. Happy travels!