Summarize this article in one sentence.
Draw a circle in permanent marker on the aluminum piece. Afterwards, create a starter hole within the circle by hammering the back of a straight-blade screwdriver into the metal. Then, place your tin snips into the opening. If you're cutting counterclockwise, use red-handed offset compound snips; if you're cutting clockwise, use green-handled snips.  Avoid straight-cutting snips—even if you manage to cut a circle, the hole will end up having a jagged edge. Use both green and red snips for tough cuts. When 1 pair stops working, swap snips. Air conditioning and heating work will most likely require you to alternate between the two because they require a combination of straight and curved cuts. Always open your snips as wide as possible before making a straight cut. Straight cuts are best accomplished using smooth, long strokes. As you cut, pull the cut strip upwards, and then to the side—this prevents it from sticking to the handle of your snips or blocking your handle during the cutting motion.  The longer your straight cut, the longer your strokes should be. Compound snips are designed for situations that require maneuvering as opposed to straight cuts. If you have no other option, be sure to fully open and close the snips for each stroke that you make—this will ensure maximum cut length. Be sure that the thick aluminum is straight and deep within the wide-open jaws before you make the cut. Although they aren't ideal for cutting curves, these snips are perfect for thick metals as they provide a great deal of leverage compared to other snips.  Straight-cutting compound strips are most effective for doubled-up or thick sheet metal. They can typically handle a maximum of 18-gauge mild steel, which is equivalent to 0.0403 inch (0.102 cm) thick aluminum.  Avoid using straight-cutting compound strips for cutting curves.

Summary:
Cut circles in aluminum using curve-cutting snips. Purchase large tin snips to make straight cuts in aluminum sheets. Use straight-cutting compound snips opened wide for thick metal.