Draw out the general design of your chair on graph paper, including dimensions. Keep your first metal chair as simple as possible, with four even legs, a seat and a back. Measure a simple chair that you have around the house for guidance.
Measure four legs, and use the abrasive chop saw to cut each leg out of the rectangular tube. Cut four tabs out of the sheet metal that measure to the empty space on the top and bottom of your chair legs. Place the legs in a clamp to hold them steady as you weld the tabs to the legs.
Find a rounded template to trace the shape of the seat. Mark the tracing onto the strip of sheet metal using a silver marker. Cut the seat of the chair out of the strip of sheet metal with the plasma cutter. Use an angle grinder to smooth the corners. Weld the seat to the four legs.
Cut three or four back support rods out of the rectangular tube with the abrasive chop saw. Cut tabs from the sheet metal to fit over the hole in each tube. Cut a bar to lay across the back supports. Weld the back support bars evenly to the seat, then weld the top support bar to the back supports. Smooth any rough edges with a grinder. Paint your chair if you wish with a water-based industrial paint.