Activate Ashnod's Altar by sacrificing Body Launderer, adding ({C} magic symbol) ({C} magic symbol) .
When Body Launderer dies, its second ability triggers, returning Grist, Voracious Larva from your graveyard to the battlefield.
When Grist, Voracious Larva enters the battlefield, it triggers, causing you to pay ({G} magic symbol) to exile it and return it to the battlefield as Grist, the Plague Swarm with six loyalty counters due to Doubling Season.
Activate Grist, the Plague Swarm's third loyalty ability by removing six loyalty counters from it.
Grist, the Plague Swarm is put into your graveyard as a state-based action due to having zero loyalty.
Resolve the Grist, the Plague Swarm ability, creating two token copies of each creature card in your graveyard that are 1/1 Insects.
Put all but one of the Grist, Voracious Larva tokens into your graveyard due to the legend rule.
When the Grist, Voracious Larva and Hidden Herbalist tokens enter the battlefield, each Hidden Herbalist token triggers once and each Grist, Voracious Larva token triggers for each token created this way.
Resolve all Grist, Voracious Larva triggers, choosing not to pay.