When I do this design, I’ve used two methods.
The first uses a center pole stand, with acrylic plates and spacers. It is a hassle, because you have to build your cake then cut a core in the center to be able to thread the cakes/plates on the center pole. The advantage of this method is that you are essentially floating the cakes and the florist can use either styro rings with flowers embedded in them (you thread the styro rings alternately with the cakes) or the florist can just stack the flowers in the spaces. It depends on the type of flowers. When we were using roses, the florists liked to use the styro/wreath set ups because they could really get the roses in very tightly. But that look has sort of been replaced by a more casual arrangement of roses and other flowers.
Now the way I do it is to use the clear plastic separator plates and the push in pillars, also in clear plastic. I don’t have to worry about getting them back (I build this into the cake price) the way I had to worry about the stand (I had three of those stands, and now - five years later - I’m lucky if I have enough pieces to put one stand together and this particular company is no longer making the stand). Because they’re clear, they are less noticeable; it helps to remind the florist to fill the center first so that you don’t see through the flowers.
You can put each cake on a thin cardboard covered with silver or white paper to blend in - I usually go with the smallest possible size I can. For a 9” round, I’ll put the cake on a 10” round board. This way I can use double sided carpet tape on the plastic plate, and just put the cake on it.
You need to be careful when placing the pillars on the plates; you don’t want to crack the plate but you also want to make sure the pillars are straight (not crooked) and locked in. When you push them into the cakes, be careful to apply only as much pressure as you need to, otherwise you run the risk of pushing too hard, and the pillars can become displaced or start to lean. Don’t ask me how I know this. Once you put the carpet tape on and put the cake in place, it is harder to reposition the plate/pillar. You don’t want to take it out and put it in and make too many holes in the cake (because this weakens it). I hope this makes sense! If not, PM me or ask more questions