Attach a backing bar, made of copper or aluminum, to both sides of the metal being welded with clamps. The backing bar will absorb the heat you apply to the metal, preventing the metal from incurring excessive heat-treated effects.
Feed welding wire into the wire feed system, following manufacturer's instructions. The welding wire should be the same metal as the soft metal you are welding. Be sure the wire has enough leeway to flow smoothly through the wire feed system, but is sufficiently tight that it does not slip.
Strike the welding electrode against the surface of the soft metal to open up an arc, and allow a small pool of molten metal to form. Depress the trigger of the wire feed system to release wire, into the front of the welding pool.
Move the arc in a zig-zag, wide fashion. This facilitates the formation of a wide weld that is not overly deep, and will help to avoid the adverse effects of excessive heat treatment.
Pull the welding torch away from the metal when you have obtained a weld of the integrity you desire. Pulling the torch away will end the arc.