Measure the width of the tread for the first step using a tape measure.
Mark a line on the length of bullnose trim to the width of the first step's tread, using a pencil and square to draw the line perpendicular to the nose of the trim.
Set the trim against the fence of the miter saw. Hold the miter saw with your dominant hand and the trim with the other. Pull the miter blade down toward the trim to cut on the line.
Measure the width of the tread for the second step. Use this measurement to cut the next length of bullnose trim. Continue the same process for each step.