The framing consists of three 8-foot-long, pressure-treated, rot-resistant 4x4 posts, plus 1x8 kickboards and 2x4 bottom and top rails. Mark where the posts will go. For this 6- by 3-foot screen, the two outer posts are set 3 feet out from the fence and 6 feet apart. The third post is set up against the fence, behind one outer post and at the opposite end of where the screen opening will be (figure C). Dig holes for the posts, about 2 feet deep and 6 inches wide to allow enough room
for concrete. Place a cobblestone into the hole to elevate the post off the dirt at the bottom of the hole. Set the post in the hole, and while holding it in place, have a friend check that it's plumb. Secure it temporarily with braces. Pour a couple of bags of dry concrete into the hole, add water to a thick consistency, and mix it using a scrap stake. Set the other two posts in the same way. Let the concrete set overnight before removing the braces.
For the kickboard, measure between the two outer posts and cut a 1x8 plank to fit between them. Slide it into place, between the posts on the ground, and screw it to the posts. Do the same for a kickboard between the outer post and the post set against the fence. For the bottom rails, cut 2x4s to the same length as the kickboards and center them on top of the kickboards. Attach them with screws.