Your comment said roughly. roughly does not cut it when mitre two angles.
Try this, lay one board on one side of the step and another on the other side of the step. The boards should overlap at the intersection where the angle is to be.
Screw each board to itself, but not down the middle. You want your circular saw to cut down the middle of the angle so that when your done and the screws unscrewed and the scrap boards tossed, you have your angle.
The angle can be adjusted by moving each board in the direction of the steps you have.
This can be your template and use it on other steps if the angle is the same.