Select the appropriate torch tip, which for aluminum, is one size larger than the tip that would be used on steel. For instance, a .040 steel sheet would require an 00 (double ought) tip. When working with aluminum, move up to a 0 tip.
Clean the parts to be welded with a stainless steel brush.
Open the oxygen bottle fully, and crack the acetylene container slightly, until you can barely hear acetylene escaping.
Use the striker to ignite acetylene. Add more acetylene until the flame is about to separate from the tip. When it is about to separate, reduce the amount of acetylene slightly. Slightly open the oxygen pin valve. As oxygen is added, the flame will turn bluish.
Apply the flame to the aluminum parts. The metal will begin to glow. When a small pool of molten metal appears on both of the parts, use the flame to stir the pools together to form the weld. Slowly move the flame along the weld line until the entire weld line is complete.
Close the oxygen pin when done welding, then the acetylene valve.