1-Buy some t-shirts when they go on sale. (This time I bought mine from AC Moore and they were on sale 4 for $10).
2-Then con your friends/co-bridesmaids into t-shirt making craft day.
3-Bring some puffy paint and anything else you have on hand.
4-One of your friends will have much better handwriting than the other two. Get her to write the theme or slogan on all of the shirts. Go for something simple, like Team (insert Bride's Nickname). The other two bad handwriters will then decorate the shirt with drawings, quotes, and designs. Be sure to draw a life-like picture of the bride on her t-shirt. Include any accessories that the bride can't live with out. For example if she is really into texting, then include a drawing of her cell phone.
So that's pretty much it. You can make the bride put on her shirt as soon as you arrive at her house to kidnap her for the evening (that's what everyone does, right?)