The backdrop for your ceremony is generally your #1 place you want to spend money on your wedding flowers. This and your bouquet will be in the most important pictures from the moment you said 'I do' that you will post and hang up on your wall. The sky is the limit as far as options go for what you can have as your backdrop. Here are a few things to first consider before deciding on your ceremony florals:
What is in the backdrop and do I want to cover it or show it off?
Does the venue rent specific arches that I want to use?
Where will I be standing for the ceremony and is there a natural structure there? Does it need to be decorated further or do I like it as is?
Do I need to bring in a structure or set up to mark the ceremony spot?
Are there any religious traditions we need to have a structure for? (ie chuppah)
Are there any other places that I want to have an extra 'wow factor' such as the entryway, mantle, line, ceiling, etc.
Once you've thought through these questions, you'll know whether you need to bring in an arch, or what your arch/backdrop will look like.
From there you can search pictures of similar arches, or sketch out where on the arch or backdrop you would like flowers. For example, if you have a rectangle arch you're working with, do some searches for what others have done, but definitely don't feel limited by what you see out there. You can always sketch out or talk out with your florist what you're envisioning in your head.
Keep in mind that the bigger you do on flowers. the more it will cost. If you're on a tight budget, you might just want one or two swags on your arch. If you have more room, you could deck out the entire arch in flowers.
If you're bringing in your own arch and don't know where to start, here are some examples of ceremony backdrop structures or arches that you can do flowers on:
Flower Towers, also known as Two-Part-Arch or
Grounded Arch or Partial Grounded Arch
And other installations: