Secure the mitre box to a work table with clamps or permanently with screws.
Determine the correct angle for the cut and adjust the mitre accordingly.
Slide the wood into the box and line it up with the properly angled slots. Hold the wood steady or clamp it into the box.
Place the saw into the correct slot in both sides of the box, spanning the width of the wood.
Saw downward with a firm but moderate pressure. Allow the saw to do the work for you.
Keep the saw from drifting out of the channel, especially with an inexpensive plastic box. It is easy to cut into the plastic instead of the wood if you lose track of where the saw blade is.
Continue to cut until the wood is completely sawed through.