It might help some if you spray starch/sizing each piece before you sew the pieces together. Then make sure each piece is cut accurately before sewing again.
Another hint is to pin the center point of the star when you put the 2 halves together, start in the center and pin to the outside edge, then sew from the center to one outside and then from center to the other outside edge. Do not overlap stitching at the center point, adds too much bulk.
Some times it helps to open the last seam and press it flat.
Remember to show us your block when you are satisfied with it.