Clean the edges of the brass tubes using the sandpaper. Wipe the ends of the tube with a clean dry cloth. The rough surface provided by the sandpaper will help the filler material adhere to the work piece.
Arrange the two pieces of tubing to be welded in a jig or clamp.
Don the respirator and other welding safety equipment. Set the TIG welding rig to a setting appropriate for the material. Connect the TIG welding rig to the work piece and start the arc. Use the arc to heat the joint area until it starts to glow red, but avoid melting the brass.
Stroke the filler rod along the hot brass surface, but do not insert it into the arc. The heat of the joint itself should melt the filler rod and build up the weld.
Continue heating the joint area and adding the weld around the pipe until the weld is complete.