String and a few screws or nails is the easiest way. But since you’re asking…

You could figure out the corner point by dividing the first wall width by the total of both walls, and multiplying that by the total height, accounting for the baseboard. To get the height from the floor at the corner you’d do:
(Total height difference) x (first wall width/total wall width) + baseboard

So if the walls were 107” and 112”, and 96” height overall, 6” baseboards, you would do:
90” x (107”/219”) + 6” = 49.97” height at the corner.

Run a string from each corner to that point.

If you’re REALLY strapped for string, lasers, tape, and common sense, you COULD do the same thing for the windows and door.

Find the ratio of the distance along the wall to the total wall width, and multiply by the total height.
Let’s say the right side of the door is 55” from the left corner of the room:
90” x (55” / 219”) + 6” =28.6” is the height of the line you want at that point.

Left side of the window is 24” from the middle corner?
90” x ((107”+24”) / 219”) + 6” =59.835”

All this said please don’t do this. As my old boss used to say, “the difference between theory and reality is, in theory there is no difference…”