I know I'm coming in late to this conversation. I suggest you head to the store, go through the pattern books and grab a pattern with this basic bag shape / style. Watch for the pattern sales. You can pick up patters for as little as a buck or two.
Then come home, study the pattern and revise it as needed to create this bag. You'll be looking for a pattern that has separate pieces for:
Michelle - No it does not add any 'puffiness' to the bag. It is stiff but no puff. I have used it and am happy with the results.
Someone mentioned providing stability to the bag bottom. I make a sleeve. Two pieces of fabric, cut and sewn to fit the bottom of the bag, sew three sides - leaving one end open. Then insert cardboard, plastic canvas, cut a thin board to fit, etc into the sleeve. Remove sleeve when you want a slouchy bag or when you want to wash the bag. Works for me.