I used this site to construct my first t-shirt quilt: http://www.goosetracks.com/T-ShirtQuiltInstructions.html
I sort of followed the instructions...I used 12 shirts but made my sashing wider and my border wider to balance the quilt and make it larger.
I backed mine with fleece so I would not have to deal with 3 laters. The friend I made it for loves it and won't let anyone else near it. She is now trying to bring me more t-shirts so I can make a second one if I don't happen to have a current project in the machine!! Such a true friend!!
:-) Paulette
Anchorage, AK