I use hydro picks. You can find them at florists. If there is a florist involved they will provide them and or put them on the stems for you. It's a little tube that looks like a medicine dropper filled with water and has rubber seal on the end.
I cut the stems to the lengths that you need, slip into drinking straws that are cut a little longer than the stem and insert into the cake. On flowers with larger stems I wrap very tightly with cling plastic wrap and insert into cake.
Thank you so much