I bought treated 1"x6" and 1"x4" lumber in 8' and 10' lengths. I ripped 2 of the 10' into 2 pieces and laid them down to provide support and nailers for the boardwalk itself. I wanted small gauge screws to attach the boards (slats) to the laths underneath so they were less likely to split the boards; to save time, I didn't want to drill a hole for each screw. I chose 1" sheetrock screws because they had lasted decades in much earlier outdoor projects.
I planned to cut the 10' boards into thirds for the slats, so I narrowed most of the path to 38"-40" and cut the 8' boards into 2 pieces for the wider portion at the mouth of the path. When I put in the mulched path with roll roofing underneath, I put a wooden lip at each end to separate the path from the lawn. I butted the first path slat against it, then used 2 small nails to create a gap between slats thereafter. I got about 7' of the path laid the first day.