Fold the chair closed and flip it upside down. This will give you the most comfortable angle to work on the brace.
Locate the break in the brace. With wooden chairs, the brace is most likely to break where it meets the leg.
Hold the brace in position and apply wood glue to the break and the leg. Hold the two pieces in place for several minutes or until the wood glue has dried enough to hold the brace in place.
Return to the chair after letting the glue completely dry. Read the instructions on the glue bottle to find out how long the particular brand you bought will take to dry.
Fold the chair and flip it upside down to give you a comfortable working angle.
Find the break in the brace. Metal braces usually break where they meet the chair's leg, or the point at which they are welded.
Turn on your welder and apply the flame to the brace and the leg until they are welded together.
Turn off the welder and allow your chair to cool down before you sit in it again. Open the chair and sit gently when the metal is no longer hot.