If it works the same as the ones in HoB, then it takes you into CAS to add or change outfits. This means wedding outfits will be selectable from any formal outfit (or perhaps any outfit) in CAS.
How you add those outfits to CAS is up to you. Do it when creating the sim, do it at home from a dresser, do it in the venue bathroom/dressing room from a mirror/vanity, do it by trying on outfits via a closet or do it by visiting a retail lot with your bridesmaids and picking a dress from a mannequin.
I'm hoping they remembered to add cake tasting interactions to the cakes themselves (or GTW registers), and did the same with flower selection. If they did, then we'll be able to make stores for those activities too.