Here's what it looked like BEFORE as I was starting to paint stair risers. It's a beautiful stairwell- it just needed a little oomph!
Now for the trim!
First, we first laid out where we wanted the trim to go by marking each piece on our walls with a pencil. We tried a couple of different layouts before deciding on a final layout. The shortest section was between the floor and the stair landing. We essentially split that section in half and the space between boards ended up being 22". We used that same spacing for the other two walls. The height of the trim is the same height as the banisters.
You'll notice the trim is thicker than the baseboard trim so we also tapered those ends with a 45° cut. When the taper was on an incline, we used a compound miter cut. Once we had all the angles and layout, we cut them on a miter saw.
We used Liquid Nails and a trim nail gun to affix them to the wall.
Here's the back side of the stairwell.