Sew it together around the side & bottom seams then with it inside out flatten it lining up the side seam with the bottom seam. You will have a triangle sort of. Decide how wide you want the bottom to be and sew straight across the "bottom" of that triangle. Be sure to measure the same distance at the other end. Turn it back rightside out and you have a flat bottom.
If you want a stiff bottom cover a piece of heavy cardboard or any heavy material and either just sit it in or handstitch it in.
If you have more questions pm me or I'm sure others here can explain it also.