You have the option of using pre-made floral cages filled with foam, attached handles and predrilled holes for attaching to the archway.
The cages are well supported with a plastic cage, a gridded front and holes for fastening to the structures.
How big of cage you use is determined by how many flowers you plan in the design. The Tribute cage below would be perfect for a large spray of flowers fastened over the heads of the bride and groom and the smaller cages used for corner pieces.
A Corso cage is an empty plastic square that you have to cut and fill in with a half a block of Oasis foam.
Again, zip ties work well with attaching all forms of cages, since they are designed with holes in the ends for wires or zip ties.
You can see how I looped several together to attach a cage to a thicker wood archway. I always buy the longest zip ties I can find, because sometimes they have to go around a pretty thick column of wood.
If you can't find the longer ties, simply loop them together to lengthen them yourself as I did below.