Death Mountain
the forest temple
It's on the map.
beat the boss
The big key
get the boomerang.
You finish off the forest temple mini boss
After you complete the Forest Temple the twilight is no longer in that region and Link must travel to the next region of Hyrule to rid it of the twilight
as soon as you defeat the Minny Boss in the Forest Temple you get the boomerang
In Twilight Princess, the only monkey whose name is given is the forest temple boss monkey - his name is Ook.
collect all the monkeys then defeat the flower boss
In the sea temple.
cut down the hanging chest in the middle room with the gale boomarang