1. On a piece of cardboard, mark a radius equal to the width of the gate opening. Cut the cardboard to make a template for setting the gate pickets. (Save the template for setting the height of fence pickets on each side of the posts.) Precut enough pickets for the gate and make a spacing gauge. Lay the pickets on two sawhorses and attach 1 x 4 bracing to the outermost pickets, making sure their tops are even. Set the other pickets in place so their tops just touch the arch template. Allow the bottom ends to run long. Double-check the squareness of the gate before adding the diagonal brace. When all the pickets are fastened, snap a chalk line and trim the pickets with a handsaw or circular saw.
2. Use the gate to position the posts, allowing for 2 x 4 blocking and 1 x 8 boxing, and 1/2 inch for swing clearance. Install the posts, setting the posts taller than needed. After the posts are in place, level across their tops and trim them 4-1/2 inches lower than the final desired post height. Add 2 x 4 blocking to the posts about every 2 feet and where the fence rails or gate hinges will be attached. Use 1x8s to make the box, fastening the edges with progressively overlapping butt joints as shown at left. Trim the 1 x 8 box so that when slipped over the post, it will be 2 inches above the ground.
3. Slip the 1x8 box over the post. (Variances in humidity, warp of the boards, or finished dimensions may require you to back out screws and trim down the blocking.) Fasten the box to the 2 x 4 post blocking with 10d galvanized casing nails. Add 1 x 4 furring as shown, mitering the corners of the bottom trim pieces.