Purchasing wedding dress online?

You can have a friend or family member measure you! Any good dress designer who only sells online will provide extensive, explicit instructions about how to measure yourself. You can do it by yourself but I'd definitely check with someone else to be absolutely sure!

Measuring yourself isn't very hard at all, but it can be helpful to have someone else do it for you - particularly for your bust measurement, since they'll get a more accurate measurement with your hands at your sides vs you doing your own. Just make sure you get a flexible measuring tape (they sell ones made of fabric or a vinyl like material at any fabric/craft store).

Look at the specific measurements for the dress manufacturers you are considering and base your purchases off the largest size of your three main measurements (bust, waist, hip). So for example if you have a 37 bust but 27 waist and 38 hip, your bust size will be closer to a 10 in most brands, while your waist/hip measurements will be more like a 4/6, so in that case you would buy a size 10 dress and have the waist and hip taken in. You can also consider the dress style as well. Something with an open back may have more flexibility in the bust (so something designed for a 35 inch bust may still work if you are 36 inches) compared to a strapless bandeau style top. Something that is a-line or ball gown style will have more flexibility in the hip measurement compared to a mermaid or fit-and-flare style. Also stretchy fabrics are ideal if you tend to fluctuate in measurements/weight, or you have bust-waist-hip measurements that vary by more than a size or two.

I'm a street size 6, but my fit-and-flare dress was a designer size 12 and had no stretch, so wasn't very easy to sit down in or do wild dance moves in. Pretty much ALL of my ordinary clothes have stretch so wearing a fitted style in a non-stretch material was quite the change for me.