I usually get the measurement of the top layer, say 8", and make a topper in an oasis iglu holer (toppers start at $20 usually, depending on the flowers). I have cardboard circles of differnt sizes already 6", 8" etc. Set the topper on the circle and start with greens around the bottom so you know how big it needs to be, add greens, then flowers up to the dollar amount. This is easy for the cake person to set on top of the cake. Then, depending on if there are spaces, size of the space, or if the layers are on atop the other we talk to the cake person to see if there is room for iglus between the layers or if they would rather have loose flowers to tuck between. Another option is just a cascade of petals down the cake. The cake person uses frosting to 'glue' the petals to the cake. I find it always is better to talk to the cake people your self so everyone knows what they have to work with. Hope this helps.
Iggy