A rough timeline
Six to twelve months out, talk to a florist, especially for spring and fall Saturdays when florists book up. You do not need every detail yet. The date, the venue, a rough guest count and a few photos you like are enough to start.
Two to three months out, finalize the list: bouquets, boutonnieres, corsages, ceremony pieces and centerpieces. Confirm colors with the dress and bridesmaid choices. This is also a good time to ask about delivery and setup at your venue.
In the last two weeks, confirm final counts and timing. Make sure someone in the wedding party knows when flowers arrive and who gets what. For small weddings or elopements, a week or two of notice is often enough.