Heavy starch on your curved pieces. Starch before you cut your pieces. Don't stretch while sewing, these are bias, also don't iron...press only.
Mark the center of your arc on each curved piece, pin together and sew from the center out to each edge.
Avoid that wonky piecing of the square by cutting the center of the block in one piece and applique the square to the center either by hand or if you are quilting "inside the ditch on your block, then just quilt the square block to the center of your block. Hope that makes sense. Love this design!
D in TX