Mow the lawn to the shortest height possible and power-rake it. Power rakes have vertical prongs that cut the ground into grooves for good seed-to-soil contact. Alternatively, scratch the ground hard with a metal rake to loosen the surface.
Sow 1/4 to 1/2 lb. of clover seeds per 1,000 square feet over the existing grass. Mix the small seeds with sawdust or compost if it is hard to broadcast them evenly. Either substance is heavy enough to act as a carrier. They also add organic matter to the ground.
Rake the area lightly to drop clover seeds caught among the grass blades to the ground surface.
Water the seeded area daily for two weeks to promote germination and strong root establishment.