When Boros Garrison enters the battlefield, it, Thranduil, Sindarin Liege, and Kodama of the East Tree trigger.
Resolve the Kodama of the East Tree trigger, choosing not to put anything from your hand onto the battlefield.
Resolve the Boros Garrison trigger, returning it from the battlefield to your hand.
Resolve the Thranduil, Sindarin Liege trigger, creating a token.
When the token enters the battlefield, Kodama of the East Tree triggers, putting Boros Garrison from your hand onto the battlefield.
When Boros Garrison enters the battlefield, it and Thranduil, Sindarin Liege trigger.
Repeat from step 4.
This clever loop turns the self-bouncing nature of Boros Garrison into a perpetual motion machine fueled by Kodama of the East Tree and Thranduil, Sindarin Liege. Every time Boros Garrison re-enters the battlefield, it bounces itself back to your hand while Thranduil, Sindarin Liege triggers its landfall ability to generate a 1/1 green Elf creature token. That newly created token immediately satisfies Kodama of the East Tree's triggered ability, allowing you to seamlessly drop Boros Garrison right back onto the battlefield to restart the cycle.