First, make a template for the body of the bag; you'll need a friend for this. Either have a friend hold the cardstock up to your back and trace the shape, or you do the same to your friend (it helps if the friend is about the same size as you). You are making a rectangle with smooth rounded corners. Don't worry about symmetry right now; we've got a trick for that in the next step.
Mark a 1/2" seam allowance around your template, then cut it out. (I didn't do this and had to add seam allowances to my fabric before cutting, and it was a pain in the neck.)
Roughly sketch a diagonal line across your template to represent the top edge of the front piece. Sketch another line 1/2" above that for your seam allowance.