Anatomy of a Proper Notch
The rule of thumb is to make the depth of the notch one-fifth of the tree trunk's diameter. The goal is to make the angles as shown in the diagram (or as close as you can). The felling cut should meet the point of the notch. When the tree starts to fall, the hinge will help guide the tree to fall in the desired direction.
Plan the Notch
You're going to be cutting a notch on the “fall” side of the trunk. Sight along the handle and adjust the saw until it's pointing toward your fall direction. The spot where the bar touches the bark will be the center of the notch. Before cutting, lay out the notch by marking with chalk or by scoring the bark with the chain saw. Make the notch at a comfortable working height. (You can always shorten the stump later.)
Cut the Notch
Make the top cut first and then the bottom. When you're making the bottom cut, adjust your hand to control the throttle with your thumb. If you meet the top notch perfectly, the wedge will drop out of the notch. But most likely you'll have to extend the cuts from either the top or the bottom so the wedge can drop free.