Activate Overgrown Estate by sacrificing Golgari Rot Farm.
When Golgari Rot Farm dies, Toph, Hardheaded Teach triggers, returning Golgari Rot Farm from your graveyard to the battlefield tapped.
When Golgari Rot Farm enters, it triggers, returning the artifact cast in step 1 from the battlefield to your hand, as it's a land due to Toph, the First Metalbender.
Resolve the Overgrown Estate ability
Repeat.
This clever loop transforms your artifact setup into a self-sustaining engine by letting Toph, the First Metalbender redefine your nontoken artifacts as lands, which turns Golgari Rot Farm into a repeatable bounce mechanism. Whenever you recast that free artifact, Toph, Hardheaded Teacher triggers its earthbend ability to animate Golgari Rot Farm into a creature, allowing you to feed it to Overgrown Estate for a steady stream of life. Thanks to Toph, the First Metalbender's continuous land-type transformation, Golgari Rot Farm's entry trigger conveniently picks up your recast artifact right back to your hand, while Golgari Rot Farm's own death trigger—fueled by Toph, Hardheaded Teacher—seamlessly sneaks it back onto the battlefield tapped to keep the cycle going infinitely.