Measure the width of the stairway at the tops of the stringers. If the stairway has a wall along either or both sides, measure to the wall. Otherwise, measure to one inch beyond the stringers.
With your pencil, mark the 2-by-10 board at the measurement you took. Use the tri-square to make a straight line across the board at the mark. With your circular saw, cut the board at the line.
Take your newly cut tread and lay it lengthwise on top of the remaining portion of the board, with the ends flush to each other. With your pencil, trace a line along the other end of the tread, onto the surface of the remaining board, to duplicate the length of the tread. Remove the tread and cut the board at the line. Repeat until you have as many treads as you need.
Set the boards in place on top of the stringers, and secure them using the drill and wood screws, drilling straight downward on the face of the tread into the top edges of the stringers. Use three screws on each end of each tread, arranged in a straight line. Sink each screw just below the surface of the wood. If there is a center stringer, screw the tread into that as well.
Run the belt sander along the front face and upper edge of each tread, to smooth the front and remove the sharp corners. If the sides of the treads will be visible, sand the sides as well.
Measure for placement of the trim, which will tuck into the rear inner angle of each step, where the risers and treads meet. Cut the trim pieces to size and nail them to the stairs with trim nails. Sink the nails to just below the surface of the wood with the nailset.