The DU Lounge

Except in the sense that Wile E Coyote was a super genius.

I would first line the stair board up with vertical corner line. Cut straight down (across) the stair side trim board so that board now has an acute angle at the bottom of the stair and the vertical corner. Mate that with the horizontal trim. Probably use a coping saw for the cut. The stair side trim board will need a portion of the bottom angle cut flush to the flat section. I would concentrate on the stair side before the horizontal trim piece. Horizontal just seems easier to match up. The acute angle can be measured using on of those angle things that lock into place. Draw a line on the stair trim and cut it with a 45 degree bevel. (Edit) make the bevel if you are not using a coping saw That’s what I would do