Cut the piece of molding on the first wall at a 90 degree and mount it flat to the wall (this will make the cut square into the corner for that piece). The cut on the second molding piece will have to be “coped”, which means you make an inside mitre cut and then cut away the profile of the molding with a hack saw (you may have to fiddle with this cut for awhile to make it exactly right, but even the pros resort to a bit of sand paper now and then!). The profile of the second piece will then fit snugly onto the face of the first piece in the corner. Here is a web site that shows how to cope a molding piece. Look at figure K. The other reason you want to cope the cut on the molding is that when the molding shrinks over time (and it will), the coped cut does not have the same separated appearance that a 45 degree cut and develop. Have fun. http://www.hgtv.com/hgtv/remodeling/article/0,,HGTV_3659_1610068,00.html