Clamp a backing bar, made of copper or aluminum, to the thinner side of the steel. The backing bar will absorb extra heat incurred by the steel, reducing the heat-affected zone of the steel.
Set the welding current to a level suitable to melt the thicker layer of steel. The welding tool will often have a brochure with recommended current levels; if not, consult the tool manufacturer.
Strike the welding electrode on the steel, to open an arc. Move the tool quickly over the thinner part of the steel and more slowly over the thicker part. Use a winding motion, instead of traveling in a straight line. A winding motion will help ensure the thin metal is not penetrated too deeply, while the thicker metal, due to the current, is sufficiently penetrated.
Allow the metal to cool, slowly, for the next few hours.
Use a grinder to remove any excess steel from the surface.