Eight pieces of 30 x 5 mm steel bar need to be bent into a curve. It doesn’t take much effort to bend the bar but the curves need to be consistent. Without special equipment, I have found one way is to bend the bar in the jaws of a vice set with a slight gap. This acts as a check to limit the tightness of the radius of the bend. I feed the bar into the jaws from the left, a couple of inches at a time. I put a small bend in the bar against the left hand edge of the jaws then feed the metal into the gap between the jaws. If it won’t fit, I bend the bar back a bit till it does. I continue like that until the bar is bent throughout its length. Any slight irregularities will not show up. I lay the bars together and adjust the bend until they all match more or less.
I bought the steel in 2m lengths as, after waving my steel rule about near the top of the pergola, I guessed that would be about right. The steel going to the corners will need to be longer than that going to the sides, so I made the corner steels the full 2m. I drilled holes near the ends for fixing. Then I screwed the steel pieces to the corners.
Then I nut and bolted the octagon in place. I then trial fitted one of the bars that attach to the sides so as to gauge how much needed to be trimmed off.