Fill in More Flowers from the Sides
Your criss crossed stems should be getting a little more crowded. You should be to a point where you can use slightly shorter stems, inserting the under the flower heads from THE SIDES of the bouquet.
Watch where those stems land - sometimes you can inadvertently have them poking out from the opposite side of the bouquet (above the lip of the vase!) You want to insert a flower stem from the side, taking care that the stem slides down into the vase, rotate the vase, insert another flower stem, rotate the vase and continue.
By now, your focal and secondary flowers are probably already in place. If you've been alternating flower varieties, you should have a good mix of flowers by now.