The photograph shows a broken wood T11 panel and is to be replaced with a fiberboard panel. Because fiberboard is difficult to cut (and expensive): The goal is to correctly cut the new panel the first time.
What methods and tools are available cut the panel correctly the first time? Panels are standard 4 ft width.
Thanks to @WayneWerner for the flashing photo / suggestion. This seems to be what is needed for the long term:
A template from cardboard or anything else could be made to the exact shape you need. Plastic template strips exist, it's how new stone countertops are cut. They're made out of the stuff you make plastic yard signs with.
Your application doesn't look too hard, but glue something together that fits the shape and it will be hard to make a mistake.
If you want to actually learn some carpentry...
Determine your roof pitch.
On a flat area with uniform shingles, or on a gable end fascia board, level out some number of inches that's a multiple of 12 (the longer you level out the more accurate you'll be).
Measure plumb (vertically) from that point to the roof or fascia board (ideally using a second level, framing square, etc.--this needs to be precise).
Divide that number by your multiple (2 for 24, 4 for 48, etc.). Looks like a 4:12, which is common.
Transfer that slope to your sheet.
Since it's 4 feet wide, multiply your pitch by 4 (12 inch feet).
Measure from a square end that distance along the perpendicular edge. Shazam.
Of course, you could also simply stand a sheet up there, tight to the corner trim and resting on the roof, and measure the vertical gap at the raised corner.
Once you have your roof pitch you should be able to cut any piece on the house to that angle.
FYI, it's rare to see the joint between sheets follow the roof slope like that. Normally they're horizontal. You could eliminate some of the degradation on the upper sheet by cutting a new horizontal line and filling in below that point.
