Measure the length, height and any overhang on the first step.
Draw the profile of the step on a piece of paper with a pencil, using the measurements as your guide. The profile of the step is the view of the step as it appears from the side. This gives you a shape to work with as the trim you will be applying is meant for the wall directly to the side of the step, whether it is a wainscotting or a baseboard.
Cut along the pencil line. This gives you a template for the shape of the stair trim that will be applied to the wall beside the step.
Fit this template to the wall beside the step it was created from, ensuring that it fits snugly. If you are confident that all of your steps are uniform, use this template to cut your trim. If your steps vary slightly from each other, make templates for all the steps.