Measure from the top rail along the fence posts to the desired top height of the pickets. Mark the height on the posts with a carpenter's pencil. Tightly stretch a stout cord between the posts and attach at the marks on the posts.
Start at either end of the fence and set the first picket in place. Use a carpenter's level or framing square to position the picket 90 degrees perpendicular to the fence rails. Align the picket with the height line and nail to the rails with aluminum or hot-dipped galvanized 8d nails.
Cut a board to the thickness of the picket spacings. Hold the board along the edge of the last attached picket. Lay a new picket along the opposite edge of the board and nail the picket in place. Remove the board from in between the two pickets and repeat this process until all pickets are attached to the fence.