Flip the sofa over so that it is resting on its back and the bottom and legs are exposed with the help of another person.
Identify the cracked angle support on the sofa leg. Spread the cracked angle apart and squeeze wood glue into the crack. Press the angle back together.
Clamp the angle support on the broken leg with a wood clamp and tighten it. Wipe off any wood glue that oozes out with a damp cloth rag. Allow the glue to dry for 24 hours and remove the clamp.
Sand the repaired area of the leg using a fine-grit sanding pad to remove any remaining glue from the leg.
Center a metal brace over the repaired crack. Drive the screws that were included with the brace through the mounting holes into the leg with a screw gun.