I use a cad program (like sketchup) to give me the cut points for the stringer (boards) on the horizontal (step 7 goes from 58.75" to 67.125"). You then use a carpenter square and they make little brass screws you can screw on at the correct rise and run. Then you replicate that triangle down the stringer, pull out a circular saw and start cutting (carefully)!

You need an angle finder to measure the angles between all relevant surfaces. Then cut the wood according to that.