There are several ways you can do this; saw, chisel, rabbet plane, shoulder plane.
Is the rebate on the roof or the panel. If it’s on the roof then make one as you would normally along the edge but if it’s along the panel then cut a 45 to fit not internally but externally to be flush but i wouldn’t do it that way as the end grain is ecposed and aesthetically doesnt look good and then rebate along the edge as you normally would. Nothing changes in operation only the angle cut.
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