When the petals are hard enough you can start to place them onto the base
board and around the cakes. Attach them as you go with a little blob of
icing.
Finish the cake by topping the top tier with as many petals as you
feel you need.
If you like you could pipe a dome of icing into the middle of
the top tier to give it additional height and than cover it with petals.
Alternately
you could place a wedding
cake ornament on top and arrange the petals around it or place a floral
ornament on top.
Here is the finished cake. A cascade of freshly made icing
petals.