I'm trying to remember back to when I was a kid, but I believe you can wet the frame with clear dope to adhere the tissue, then spray the tissue with water to shrink it tight. I'm sure someone who knows more about it or a better way will post soon.
Here are a couple of links with good info.
This one uses the old method I remember:
http://www.antiquemodeler.org/Comet_...ing_Guide_.htm
This one uses Elmer's glue to attach the tissue and rubbing alcohol to shrink the tissue:
http://findarticles.com/p/articles/m...02/ai_n8942705