I found this in Southern Living Magazine Christmas something or other. They claim you only need one magazine but with the magazines I used I found the trees were too sparse (the one exception was the Dec 2010 NatGeo). Find 2 magazines that are the same size, two issues of the same magazine work if you have find them.
Working front to back fold the top corner down, getting as close to the spine as possible.
Fold down to the spine again