First cut two rectangles of the outside material. We cut around an existing Oyster Card Holder. Make sure you leave extra for seam allowance.
Next cut a rectangle the same size of a contrasting material for the slots.
Now cut the contrast piece in half and hem one end of each piece.
Iron some interfacing onto the wrong sides of the outside material.
Now you need to sandwich the pieces together. Put the first outside piece right side up, lay the pocket pieces right side up on top and place the other outside piece wrong side up on top.
This is when you should also add the elastic or ribbon to secure the holder too. If using ribbon make sure you leave a bit extra by pinning it in the middle so that it will reach when it's finished.
Sew round the edges leaving one open for turning and trim the excess
Turn the whole thing right side out and hand sew the open seam.
Give the whole thing an iron and you're done :-)