As first do the edges of the circle and both longer sides of the reclangle. Then I sew the circle with one longer side. Cut the shorter seams to be of the same size and sew them together. Now you can do these edges as well.
Fold the upper part and sew it (I do this twice). I recommend to make a piece for the string rather wide, it's easier to tie the sack. Don't forget about a hole for the cord:-) on the inner side.
P.s. : if any of you knew what's the damned name for making jagged machine stitches on the seams, please let me know, none of my dictionaries does :-[