HOW can this be so hard?? Please help my brain understand how to measure and cope this joint

Finishing (This is an angled roof/ceiling to a wall)

https://preview.redd.it/t0y1eqricxoa1.jpg?width=640&crop=smart&auto=webp&v=enabled&s=c8c52ec698a5616cc5de4b72a74c4e81aa74f74b

https://www.reddit.com/r/woodworking/comments/11wgtvq/how\_can\_this\_be\_so\_hard\_please\_help\_my\_brain/?utm\_source=share&utm\_medium=web2x&context=3

#stolarstwo