Summarize this article:

Wear a long-sleeve shirt when operating a chainsaw. You should also wear safety gloves, safety goggles, ear protection, a hard-hat helmet, and chainsaw chaps. Each of these items is important: they protect you from injury while operating a chainsaw. Chainsaw chaps are heavy aprons or pants made of layers of protective fabric, to be used when operating a chainsaw. You can purchase chaps online or at a hardware store. Make sure there is nothing in the area where the top of the tree is going to fall. Clear away any items such as dog houses, bird houses, fences or children's play equipment near the area where the branches will fall. If you are cutting down a whole tree, make sure you have a clear escape route so you can move safely out of the way as the tree topples. Not all trees can be topped without professional help. If there are immovable objects in harm's way, (large structures, for instance, or power lines), consult a tree-care professional. Make sure you're standing comfortably.  Lay the chainsaw on a flat surface (such as the ground). Place your left hand on the handlebar and your right hand on the starter-rope handle. Pull the starter rope gently until you feel resistance. Then pull the rope toward you several times until the engine starts. (Consult the manufacturer's instructions for advice on using the choke mechanism.) Once the engine is running, briefly pull the throttle trigger to rev the engine (to make sure it will successfully accelerate). Then carefully lift up the chainsaw. Make sure you have secure footing when starting a chainsaw. If you need to use something like a ladder to top a tree, it's not recommended you do so alone unless you have extensive experience operating a chainsaw. Stick to topping shorter trees where the branches can be reached when standing on the ground. A Humboldt cut is a three-step process for cutting down a tree (or part of a tree). First cut into the trunk just above the highest branch you want to retain. Place the chainsaw against the side of the tree toward which you want the top to fall. Make a horizontal cut into the tree. Stop cutting when you're about 1/4 to 1/3 of the way through the tree's diameter. (In other words, stop cutting well before the chain reaches the center of the trunk.) After making your first cut, move your chainsaw down several inches. Angle the chainsaw upward and make a diagonal cut that connects with your first cut (again stopping about 1/3 of the way through the trunk). You're cutting a small, horizontal wedge out of the trunk.  A small chunk of the tree will fall to the ground after you make this second cut, leaving a gash in the tree's side.

Summary:
Put on safety gear. Clear the surrounding area. Start your chainsaw. Start with a horizontal cut. Add a second cut below the first one.