Measure the full back width of the chimney (points A to B). Mark a point in the center of the chimney 6 to 12 inches above where the chimney meets the roof (point C). Holding your measuring tape level, extend it from point C to where it meets the upward roof slope (point D).
Measure the following two triangles and cut them out of the plywood: A-C-D and B-C-D. Line up triangles with sides C-D facing, and attach them with L-shaped brackets. Tighten screws enough to let you handle the cricket as one piece but loose enough that you can make minor adjustments in fitting when you place the cricket behind the chimney.
Cover the cricket with leftover shingles or roofing membrane, leaving extra flaps of shingle or fabric that can be layered with other joining and flashing materials. Tuck flaps under shingles or metal flashing and cover with additional membrane before applying flashing adhesive. This process may be a bit different from one chimney to another, depending on materials previously used, but it best resembles the steps needed to re-cover and secure the roof of a dormer.