I think this first one is all that you will need but I put in the other one for you to look at.

http://www.sewi…

http://www.henr…

There are so many design possibilities that I'm going to suggest you might want to start at your library. I'd particularly suggest Nancy Restuccia's book, "Hold it!", which has a lot of diagrams and instructions for many different types of bags. Although she gives dimensions for her bags, you're certainly free to make a bag to the dimensions you need/want. It's an older book, so should be easily available on interlibrary loan.
http://www.worl…

Here's her basic tote bag, so you can get a flavor of her instructions: http://www.mcke…

Here are several to choose from - http://tipnut.c…