Clean the aluminum. Remove oxides with a stainless steel brush, and use a degreaser to remove surface oils and impurities.
Preheat the aluminum to 300 to 400 degrees Fahrenheit in an oven. Preheating will make it easier to move your welding tool quickly throughout the aluminum, and increase the evenness of heat distribution.
Clamp the metal pieces to be welded together. Since aluminum has a tendency to preserve spaces between materials, clamping will help you to avoid holes in your weld.
Attach the aluminum to a backing bar, ideally made of a metal such as aluminum or copper. A backing bar will act like a heat sink, absorbing heat from the aluminum to prevent it from overheating.
Increase travel speed as you weld, since the rate at which aluminum heats up will increase as you move through the material. Make short welds throughout the length of the material, spaced evenly through the material. This method of welding, called skip welding, will minimize the heat-affected zone of the aluminum.