This stairs use to connect first to second floor, only one way, so I take around the building and make a sketch for the first design.

Of course, I need to design in 3d view, I want to see every angle of stairs. For this, I use SketchUp on web.

Let's make it more clearly. All details have designed. With this drawing, I can calculate how much materials I need.

First, I've made main bearing structure. Most of herringbone stairs use standing pillar to support stairs, for me, it's not pretty at all, I want the steps to fit into the main load-bearing structure, less is more. 
I've weld main structure first, then use truckle to pull it up in position. This is most heavily job that i have to do, I tight the structure with rope and rotate the truckle. Finally here:

Keep finish main load-bearing structure left. Then welding each step on ground then bring it up like image below, step by step.

Cover all structure with black color. I've use spray machine, also cover wall by paper.

Time for cutting some wood plank for step.

I use drill to mount wood plank to steel structure.