This is a simple project that should only take a couple of hours to complete.
Start by clamping the two 12' - 2x4's. The rungs are notched into the 2x4 upright 1-1/8" so we need to mark out notches. The top and bottom rungs are 8" from the ends. This is the top of the upper rung and the bottom of the lower one. Offset these marks 2-1/2" (the width of the 2x3 rungs).
Now measure out for the rest of the rungs. Lay your tape measure on the 8" mark made earlier and measure 16" increments. These marks represent the center of the intermediate rungs. Offset these marks 1-1/4" on either side.
Square off the marks you have just made down the face of the 2x4 uprights. Square your marks across the top edge of the uprights as well.
Now it is time to cut the notches. Set the depth of your circular saw to 1-1/8". Start by making the cheek cuts of the notch then proceed to make a series of cuts about a 1/4" apart through the rest of the notch. Do this for all the notches.
Before removing the clamps use your hammer to break out the wood in the notches and clean up each notch with your chisel.
Unclamp the two uprights and set them aside. Now cut all the 2x3's into 19" lengths. Each rung is attached to the upright with exterior wood glue and deck screws.
Now attach them to the uprights starting with the top and bottom rung. Put some of the exterior wood glue into both notches and insert a rung. Make sure the rung in flush to the outside of the uprights. Secure in place with 4 deck screws. Continue this for the reminder of the rungs.
Protect with paint or stain and this ladder will last for years.
About the author: Dave Markel has helped hundreds of individuals improve their wood working skills. Visit his site at Willowbank Wood Working Plans