Lay a straight edge (yellow) close to wall at corner and mark a line A-B.<br>Lay a straight edge close to adjacent wall, use a square to make perpendicular to A-B and mark a line A-C.<br>Mark point D so that A-D = B-C, and check by measurement that A-C = B-D and A-B = C-D<br>Mark lines C-D and B-D<br>
