The post locations shown are for a 12x12 deck layout as decribed in this plan. Consider pre-cut post when purchasing your decking lumber. To make the notch cuts, use a circular saw or table saw to cut to the lay out lines. Use a handsaw to finish the cuts. To install the post set the post in location.
Pilot drill two holes through the post and joist.
Be sure to hold the post plumb. With the post now installed nail the top and bottom rails to the inside of the post using 16d nails. Keep them level and spaced as shown at the top and bottom. The gap must not exceed 4 inches. You should miter the corners for a more attractive appearance.