next step is to press the second petal open, then lay the third petal face down, lined up on the center. just as you did the first and second
as i hope you can see from the next set of photos, you'll go all the way around the flower that way. always center the petal face down on the center, then sew from the outer edge in toward the previous petal
use straight-pins to anchor the petals and to make sure you don't sew past the corners.
if you do sew past the corners, you really need to pick away the extra stitches using your seam ripper