Simple! Just use some 1x2's or 1x4's and use wood screws to put them up on the wall on all four sides to the inside of the 1" square aluminum frame. Then you can easily place the frame on the wall and you know the frame is level. Also, the wood will keep the frame from bowing, as the screen is stretched VERY taught when snapping onto the frame.
Believe me... you're in for a real treat when it comes to snapping the screen material onto the frame. Their is a reason that they have sewn-in pull handles on the corners.I can remember saying a few "not so nice" words when I was attaching my old Draper screen material.http://www.avsforum.com/ubb/wink.gif
------------------DManThe Academy Home Theater