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.
The Lost Scrolls of HANDWORK
(Hand tool only woodworking magazine)
I think I’d do it the other way round. Start with the rebate and plane/chisel a 45 on the narrowed section. If you gauged for the thickness of the rebate you can use that same setting on the end of the piece to mark the 45. Then use that same gauge setting registered against the end of the mating piece and the outside face to mark out for the 45; cut away that section square, then using a rebate plane or chisel take off the corner to make the mating 45.