There are a number of tutorials out there and the norm is that one adds an inch on the dimensions each way on the item and you can figure out the measurement of the fabric required. The ipad 3 is 7,5" (19 cm) x 9,5"(24 cm) - ergo I cut out 8,5" (22 cm) x 10,5"(27 cm) fabric pieces: outside layer, cotton batting and lining.
Velcro shreds this fabric somewhat and I did not have any fancy buttons to use as closure, but instead used some lovely ribbon I happened to have. Doesn't it look like a present with the bow on it?
As you can tell it is a bit short, so that 1 inch extra rule did not work for me. Perhaps 1,5" is more appropriate for the length. In any case, it fits well into my purse now and I no longer fear getting it scratched.
Have a brilliant day!