Put on protective clothing, gloves and eyewear.
Turn the barrel over so it is standing on its top end; the top should have a small hole used to fill the barrel. Drill three evenly spaced holes in the bottom of the barrel. These will serve as drain holes to let rainwater out.
Set the barrel on its side.
Light the torch and, holding it at or near a 90-degree angle to the barrel, cut along the top rim to remove the top of the barrel. Turn the barrel slowly until you have cut all the way around. Discard the barrel top.
Cut three evenly spaced holes in the lower section of the barrel. Each hole should be 3 inches square, and 1 inch or 2 inches from the bottom edge of the barrel.
Light the torch and adjust it to a low heat setting. Spot weld each of the three pieces of metal screen to the interior of the barrel, so they cover the vent holes. Be careful not to cut through the side of the barrel.
Begin fabricating the square frame for the top screen. With the torch, cut three 90-degree notches in the angle iron, spaced 25 inches apart.
Cut each end to 45 degrees, mirroring the angles so the ends will fit together neatly. Using the torch, heat the outside of each notch. Hold the cool ends of the metal and bend each angle to 90 degrees. Let cool, then repeat with each corner until you have a square frame.
Weld the two ends together to solidify the square frame. Place the large piece of expanded metal inside the frame and weld it into place.
Place the screen on top of the barrel before you light your first fire.