1

Notch the tops of the posts. Cut as deep as you can from both sides with a circular saw, then finish the cuts with a handsaw.

2

Mark the brackets without fussy measurements or geometry—just align a framing square with the edges of a 2x10 and make three marks.

3

Draw perfect curves fast using a tape measure to guide your pencil. Cut out the bracket and use it as a pattern for the other bracket.

4

Screw through the posts and brackets into the header. That way, one header will have no visible screws. Screw through the second header into the posts.

5

Set the arch level and plumb before you pour concrete into the postholes. Wedge shims under the stretcher until the header is level, then plumb and brace the posts.