Soffit is the horizontal panel that extends from the top of your wall to the back of the fascia, or edge board at the bottom edge of your roof. For soffit to be installed, a horizontal fascia board must be attached to the ends of the rafters. To do this, the rafter tails must first be cut with a vertical face at the bottom end, to attach the fascia board to. The cut is simple enough to make with a circular saw, but you must first find the right angle and mark the rafter tail.
Place an angle finder at the peak of one gable of your roof, with one leg of the finder aligned to each angle of the roof, going down from the peak. Read the angle from the finder and divide by two. This is the angle needed to create a vertical face on the end of your rafters.
Use a quick square, with rafter angle marks, to mark the ends of the rafters. Position the triangular square with its diagonal edge toward the wall, and the square corner opposite the diagonal aligned with the bottom corner of the rafter. Press the lip of the bottom edge of the square up against the bottom edge of the rafter. Locate the correct angle in the diagonal slit in the middle of the square and make a mark aligned with it.
Use the square as a straight edge to draw a line from the corner, through the mark to the top edge of the rafter. Set the circular saw to its deepest setting and cut along the outside of this line. Mark and cut every rafter to this same angle.