I will really have to get hold of a digital camera and show some of my bags - I made a lot of bags with granny squares. To get it more the right shape for a tarot deck I added rows on one side. It makes a strong bag, and to make extra strong bags for decks that don't have a box, I add cotton lining.
With this kind of pattern you also automatically have holes to pull the drawstring through.
It is easy to mix lots of different colours so I can still make a bag fitting for one particular deck with the same easy pattern through my choice of colours.
I really love that heart-shaped bag Solitaire posted.
I agree about reading patterns, my gran was a grandmaster in crocheting but she could never read them, she looked at the charts and made it all after those. But yes, reading as you go along will sometimes help.
Good luck Kat, let us know how it goes
A site with granny squares:
http://www.bevscountrycottage.com/grannies.html