You can draw it on a paper template, then stick it to your screen.
Or measure it out in the screen from a known datum point using a makeshift compass (string and marker pen).
Or use a decorators profile tool.
Done a few of them now in different forms, shapes and curvatures. Just make sure you measure twice, cut once. And don't be afraid to get the wood saw out. I'm deadly serious, as long as your gentle, it leaves a cleaner cut than hack saws or rotary blades as it doesn't heat up the material to melting point.