Disclaimer: I hate math and try to avoid patterns when I can; a lot of my designs are free handed and imprecise.
Outside and lining fabrics
A medium to large sized button
Needle and thread and/or a sewing machine
For the body, cut two 1' squares out of both the lining and outer fabric each (you should have four squares once all cut)- leaving about 1/4" of seam allowance for the outside fabric. For the outside fabric, pin right sides together and sew up three sides, leaving top unsewn. *If you have directional fabric you will need to consider which sides to sew.The lining fabric needs to be slightly smaller so skip the seam allowance. Before you can sew these squares together you'll need to attach your pocket. (See below.) Once pocket is added, pin right sides together and sew around three sides, leaving top unsewn.
For the pocket (if desired) cut two 4"x4" squares out of your outer fabric (or lining fabric, it's your purse!)- leaving 1/4" of seam allowance. Pin right sides together and sew around square, leaving a small hole to turn and point corners. Turn right side out, point the corners and pin to one of the lining squares. Sew closely along the edge of the square around the 3 sides, leaving top open.
For closure you need to cut out two small, thin rectangles with rounded tops. Mine was about 4" long. (No need to leave much seam allowance, maybe 1/8"). Pin right sides together and sew around leaving bottom unsewn. Turn. If you have a sewing machine, you may have a foot and button hole application on your machine. If you don't or lost your manual like I did, you can do this by hand. (It's easy!)Take your button and hold it on it's side on the tab you've sewn where you want it to be positioned, mine was about 1/2" away from the rounded edge. Mark with chalk or wash out fabric pen. The button hole should be a bit wider than the thickness of your button. Be brave and grab your seam ripper and rip the hole that you've just marked. Take a needle and coordinating embroidery floss, simply whip stitch around the hole to prevent the edges from fraying. Tada!
Ignore how phallic it looks, but this is the general shape you are going for:
For the strap just cut a strip of you outside fabric to the length you prefer, I like long straps so mine was around 2 1/2' and 3" wide before sewing. Pin and sew right sides together to make a tube, turn right side out.
Now for the assembly:
Sew button into place on your outside fabric. Place your lining fabric inside of your outside fabric, it should look like how the finished purse will be. (Do not make the mistake of sewing the purse together with the right side of you lining facing the wrong side of you outside fabric- your lining will be inside out.Fold 1/4" in around the outside and lining fabric so no raw edges show. Pin button tab in position, as well as strap. Sew around the top of your purse- this will secure your strap and tab in place.
You should now be the proud owner of a new, simple purse. I like slouchy purses but if you want a more stiff/stable look you can use interfacing to give it structure.