Activate Selvala by paying ({G} magic symbol) and tapping it, adding at least ({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) ({G} magic symbol) .
Activate Yahenni twice by sacrificing Slimefoot and Squee and Selvala.
Activate Slimefoot and Squee by paying ({1} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) 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.