Make the arbours
Set out the arbour positions using a temporary 90 x 45mm timber frame, measuring the diagonals for square then bracing it. Mark post locations with stakes at the corners then dig 900 x 300 x 300mm post holes.
Add 100mm gravel then the posts to the holes. Check for plumb then skew-screw the posts to the frame. Clamp temporary braces between the posts and frame then mix water and fast-set concrete in the holes.
Use offcuts to support the lintel then secure with 120mm x 14g bugle screws. Continue the height mark around the post using a speed square then secure the second lintel and cut the post to length with a handsaw.
Remove the lintel supports then prop the base of the cladding above the post footing using a timber offcut. Secure the cladding, butting against the lintel with equal overhang both sides then add the inner cladding.
Leave off two sides of cladding. Position the rails side by side and square lines across the edges 223mm from one end then at 150mm centres. Secure the spacers with 40mm x 8g screws, adding the rails as you go.
Attach a straightedge 140mm above the top rail as a guide, position the slats centred on the markings and butted against the guide, then secure with 60 x 2.8mm galvanised nails. Add the remaining cladding.