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.
Use Wedges on Big Trees
If you have a tree that's more than 18 in. in diameter, go ahead and make your notch cut and begin the felling cut. Stop cutting as soon as you've penetrated far enough to pound wedges behind the bar. Leave the bar in the cut with the saw running, but lock the chain brake and tap in the wedges. Then finish the cut. Wedges will keep the saw from getting pinched in the cut if the tree leans back.