Adding in the names of the brides, and/or the date of their marriage, makes your card that little more personal.
For all of these designs, what you actually write is up to you. You could enclose a message which really says it all - an in-joke, a shared moment, a verse or a song lyric, which you know the couple would appreciate.
In order to add a personal message to your card, you just have to visit the site. Once there, select the 'Customize It!' button.
You will be directed to insert your own text (or even images, if you really want to branch out!).
The artists frequently highlight where you should position your message, which is very helpful for the non-creative people like me. You can follow their direction, or ignore it in favor of your own ideas.
Once done, simply save and continue to the checkout.
There are plenty of cards which allow room for such things. I've naturally begun with the cutesy ones, because I like them!
Incidentally, if your couple (or either bride) has a different skin tone to those shown here, explore the store. Every one of these cards can be bought with different ethnicity for one or both brides.