Mark off four inch courses from the bottom of the window to the sill and snap a level chalk line. Double up on your first course so your clapboards flare our at the bottom. Nail one inch from the bottom of each clapboard every sixteen inches into the wall studs. Make sure you use stainless steel ring shank nails to avoid discoloration. Leave your clapboards slightly long to allow for shrinking and a nice snug fit. Start at the bottom and work your way to the top.