Activate Selvala by paying and tapping it, adding at least 1.
Activate Umbral Collar Zealot twice by sacrificing Selvala and Slimefoot and Squee.
Activate Slimefoot and Squee by paying 1 and sacrificing a Saproling, returning it and Selvala from your graveyard to the battlefield.
When Slimefoot and Squee enters the battlefield, it triggers, creating a 1/1 Saproling creature token.
Repeat.
This clever loop relies on the resurrection engine of Slimefoot and Squee to continually return itself and Selvala, Heart of the Wilds from the graveyard to the battlefield, fueled by the massive amount of colored mana Selvala produces when she re-enters. Meanwhile, Umbral Collar Zealot serves as the grim catalyst, letting you sacrifice your key pieces to fuel the graveyard, while Thousand-Year Elixir provides the crucial haste-like speed needed to bypass summoning sickness and tap Selvala the moment she returns.