Unless, of course, you have been properly trained and hence have a little card that says you are competent to fell a tree. This is especially important if you plan to work trails in a national park without being arrested.
With that in mind, here is the Søren Ericsson method for felling trees:
First, pick out a tree with a clear felling path. Ideally, you should be able to fell the tree downhill and in the same direction as it is leaning.
Next, clear your escape path - 135 degrees from the felling path. Make sure it's free of brush and other obstructions. While you're at it, clear your work area as well.
Now you're ready to start on the tree. Line up your felling sight with the desired felling path. While the felling sight is aligned with the felling path cut a 90 degree notch into the tree, about 20 percent of the tree's diameter deep. Next, do a bore cut into the tree, leaving about 10 percent of the tree's diameter between the notch and the bore. This will be the "hinge" that keeps the tree going where you want it. Continue the bore cut out the back of the tree, stopping to drive in wedges to keep your saw from getting bound.
Now, take your felling lever, stick it in the back cut, and pull down hard. The tree should fall gracefully.
Congratulations! You've just felled your very own tree! Now, go ahead and limb it, buck it, send it to the sawmill, and make furniture!