Position the blade over the area where you want to start your trench.
Turn on the machine. Many trenchers are equipped with an electric starter, but smaller units may have a pull starter such as those on a lawnmower.
Tilt the blade into the ground and slowly start to walk forward until you feel a sudden resistance stopping the saw from moving forward. The resistance likely means that you have encountered a root. Make sure to maintain a tight grip on the trencher's handle at all times, because a root can jerk the saw away from you.
Hold the trencher in place as the blade spins. Slowly inch the blade forward as you allow time for the blade to cut. Be cautious -- once the blade cuts through the root, the saw might lurch forward when it reaches softer soil. Continue inching forward until you feel the resistance end.