I mapped out my stair rise and run measurements, but how do I accurately mark them onto the 2x12 stringer board?

Clamping a pair of brass stair gauges onto a standard framing square at your exact rise and run numbers is the best way to trace consistent steps down your board. Slide the square along the crowned edge of your 2x12, tracing each notch continuously, and remember to drop the bottom riser cut by the thickness of your tread so your finished step heights match. Because building codes strictly limit height variations between steps to prevent trip hazards, hire a licensed contractor if you are unsure, as miscalculated stringers won't pass structural inspection.