Bridal Shower Hostesses Thank you cards

I’d wait until after and mail them so you can thoughtfully reflect and reference specific touches that you especially appreciated (great food, a special piece of decor, etc.)
As far as what to write—a sentence or two thanking them for their overall effort and how special they made you feel, a line about a specific touch and closing it with a line on how much you appreciate having them in your life should cover it!

I'd wait until after so you can express gratitude for specific elements or events that you enjoyed most.

Mail them after the shower with each getting their own card. Write a heartfelt message in each thanking them for their generosity. It doesn’t need to be more complicated than that.

I would mail them after the shower so that you have time to think about your written messages and not feel rushed.