Weddingbee boards

I want to do something like the envelope guestbook as seen on Martha Stewart Weddings, but I want guests to use their tented place cards as the card on which they write their messages. http://www.marthastewartweddings.com/good-things/envelope-guest-book

This way one side of the card would have their name on it and it would be a double use for the place cards. I may use glassine envelopes so that the name and design of the cards can be seen in the book. I'd imagine that guests would have to find their seats first and then they would write their cards which I am fine with since it typically takes everyone a few minutes to figure out what to write. I guess I would have to have pens at each table for them to write with, but how do I let them know what I want them to do?