Selvala triggers, creating a 1/1 Mercenary creature token.
When Kor Skyfisher enters the battlefield, it triggers.
Holding priority, activate Earthcraft by tapping Kor Skyfisher, untapping the Plains.
Resolve the Kor Skyfisher trigger, returning it to your hand from the battlefield.
Activate the Plains by tapping it, adding .
Activate Earthcraft by tapping the Mercenary, untapping the Plains.
Repeat.
This crafty loop turns the humble act of bouncing your own permanents into a self-sustaining engine. As you repeatedly cast Kor Skyfisher, Selvala, Eager Trailblazer rewards your creature spellcasting by steadily populating your board with Mercenary tokens, which rapidly diversifies your creatures' power stats for massive mana production when needed. Meanwhile, Earthcraft bridges the gap by allowing those creatures—and the Skyfisher itself—to tap to untap your lands, footing the bill to recast the Kor Skyfisher over and over.