Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return
Ashaya from your graveyard to the battlefield, and attaching
Nim Deathmantle to it.
When
Ashaya enters the battlefield,
Springheart Nantuko triggers, choosing not to pay and create a 1/1 Insect creature token.
Activate
Ashnod's Altar by sacrificing the creature token, adding ({C} magic symbol) ({C} magic symbol) .
Repeat.
If you control a nontoken creature that has haste, you may instead sacrifice it, and then tap it each iteration to get infinite green mana and colorless mana.