Outline the proposed edges of the walkway with stakes and string. Mark corresponding footer points along the inside edges every 6 feet, using more stakes.
At the footer points, dig footer holes with a post-hole digger. Make the footer holes 12 inches deep with 8-inch diameters. Line the perimeters of the holes with footer forms. Position the tops of the footer forms at least 1-inch above ground. Place a level across the tops of adjacent forms to check for evenness.
Fill the bottoms of the holes with 1 to 2 inches of gravel. Pour ready-mix concrete into the footer forms, leveling it at the tops of the footer forms with a trowel.
Embed metal post anchors into the centers of the wet concrete, aligning the braces on the anchors parallel to the edges of the walkway. Allow the concrete to harden.
Construct double 2-by-6 beams for each side of the walkway by nailing together pairs of 6-foot-long 2-by-6s, using 10d nails. Stagger the nails in a zig-zag pattern so that half are about 1 1/2 inches from the top edge and half are the same distance from the bottom edge, driving a nail every 8 inches. Nail through both sides of each beam.
Set the beams into the post anchors, running them parallel to the edges of the walkway. Position successive beams with their ends butted together over the centers of the anchors. Drive the manufacturer's specified nails or screws through the holes in the anchors and into the beams. Note: If the anchors are designed for 3 1/2-inch-wide lumber, fill the gaps with rectangular shims cut from ½-inch pressure-treated plywood and nailed to the beams with two 10d nails.
Cut decking planks for the walkway surface. Cut planks with lengths equal to the space between the outer edges of the beams for flush walkway edges. Cut planks 4 to 8 inches longer for a walkway with overhanging edges. Center the flat planks across and in succession along the beams to create a boardwalk. Screw or nail the boards to the beams.