Activate Llanowar Druid by tapping and sacrificing it.
Llanowar Druid dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Llanowar Druid from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Resolve the Llanowar Druid ability from step 3, untapping all Forests.