I've loved the generosity of many in the blogging world for their tutorials, so here is my attempt at one (please bear with my writing style and some of the crappy photos...my sewing machine is literally under a flight of stairs).
This is a basic tutorial for an "envelope" type purse, which can be adapted for many uses.
The first step is to figure out the desired dimensions of the finished product. Then with mental math or a trusty calculator, calculate the fabric size following these guidelines:
width: desired width + 1" seam allowance
height: desired height x 2.5
You will need two pieces (outer and liner) and one of interfacing (cut about 1/2" smaller all around) -- I like using the heavy duty kind.
Putting the right sides together with the interfacing behind, sew around the edges using using 3/8" seam and leaving a 4" opening. Trim the corners for less bulk.
Turn inside out, making sure the corners are nice and square (I like using a big fat knitting needle) and using fingernails or an iron, press.
Evaluate which end would make the more attractive flap and then top stitch the opposite end (I usually add a shop label here).
Fold up the bottom edge to the desired height and pin in place. Starting at the bottom corner, top stitch up the side, around the flap and down the other side.
Add a snap closure and it's all done! It's great for a cash, "feminine products", eyeglasses and etc.