Then, make the curved end cuts.
Install the crossbeam pairs at the posts first. When they’re in place, bore screwholes down through their top edges and screw crossbeams to the support beams. Then add the three remaining pairs with similar spacing.
POST TRIM AND BRACES
Cut the post trimpieces to length and width. Note that you’ll need to notch some of the pieces to fit between the support beams, or you can make filler blocks to cover the post cores at these areas. Instead of trying for perfectly flush corners, we dimensioned the trim to leave a 1/8-in. shadow line, or reveal.
Secure the trimpieces to the posts with construction adhesive and galvanized finishing nails.
Use 2 x 6 stock for the diagonal braces. Cut the ends to length at 45 degrees, and use a flexible stick to lay out the shallow curve on the lower edge of each brace. Fasten the braces to the posts and beams with screws.
FITTING THE TOP SLATS
Cut the five 2 x 4 slats to length and shape the ends.
Clamp each slat in place and mark the crossbeam notch positions.
If you use a table saw and dado blade to make the cuts, be sure to support the long stock at the opposite end.