if you could take apart a bag you'd be suprised how simple the shape actually is.

the picture is a very rough and NOT to scale pattern, but it can serve as a guideline for a simple bag shape. all of the numbers are in inches. at one point i read "examine every bag you can", and it's true. when you're starting out take a look at all sorts of bags (not just messenger ones), whatever you have lying around, and turn them insides out, stuff like that.

also, muslin is a kind of cotton fabric thats usually white or off white but really cheap. you can try messing around with that first, as not to ruin any of your good stuff.