I first narrowed it down by the overall feel of my wedding. I am having paper lanterns, milkglass, and a very neutral color palette (black, gray, ivory, and white) — think clean/natural and less vintage/rustic. So that eliminated certain things like a satin ballgown.
Then, I tried on zillions of dresses. Yes, zillions ha. I even bought a few along the way, but I don’t recommend that. Just try more on. 😉
But seriously, be open to different shapes and styles. Try on straps if you wanted strapless, and try on strapless if you wanted straps. Don’t avoid a silhouette you think won’t work until you see it for yourself and then you can rule it out. I was not impressed by A-line dresses with natural waists on me, though they are supposed to be pretty much universally flattering. I felt it made me look bigger, but a dropped waist A-line slimmed my entire torso. Who knew? Then, once you have a shape in mind, consider the kinds of fabric or embellishments. I wanted lace, but the more lace dresses I tried on, the more bored I got. So I tried some tiers and floral designs and ruffles, and I loved them but I knew they were maybe too over the top for my style. So I found a dress with a striped lace pattern alternating with pleated polka dot tulle. Perfect! Just the right amount of interest without being over the top. And it’s a dropped waist, full A-line. Perfect perfect perfect! But I had to try on some ballgowns, some mermaids, some slim A-lines, some dropped waisted poof dresses, some taffeta, some pick-ups, some organza, some sashes, some beads, and everything else along the way. It was a long process for me.
But then, too, you have cases like my friend who tried like 5 dresses on at one shop and picked her favorite and was done. You’ll know when you get there how you feel about it, and trust yourself. Don’t be swayed by the salespeople or even your friends/family.
This is becoming a novel, so I’ll stop now, but ultimately, there is no right way to know — you just have to get out there and see what it all looks like on you until something catches your fancy. Good luck! 🙂