To layout and cut rafters using a framing square, align the framing square with the top edge of the rafter, and mark the plumb line near the end using rise and run calculations. Follow this pattern. Mark plumb and seat lines to complete the right angled notch that fits over the top of the wall and secures the rafter. Use the straight edge of the framing square to mark the rafter's end, then cut straight along these lines.
Make the rise and run calculations in units per foot of run, to ensure the framing square can accommodate the measurements. To avoid the framing square slipping or moving during the process of laying out the rafters, clamp the framing square onto the desired lumber of the rafter. Along with the plumb lines, mark seat cuts as well, to create the right angle notch that fits over the edge of the wall and secures the rafter to the structure. When the plumb marks are made, trim them to the thickness of the roofing ridge board to account for the difference for when the roof is constructed.
When marking the overhang of the rafter, flip the framing square over to create an opposite angle to the one used the make the plumb and seat marks. Mark the overhang to preference. Use a handsaw to cut along all of these marks. The straight edge of the framing square can be useful here if clamped onto the rafter and used as a guiding tool for cutting a straight line.