All Masks Edit
You will be a Deku Scrub at first, but to get the mask you have to go through a lot of trouble. This is actually just something that is required to beat the first part of the game, so I won't explain how to get it.
In the Goron Village, go to the edge and you will meet the owl. He will then lead you through an area with invisible platforms. Get the Lens of Truth in the cave that he leads you to. Go back and use it to find Darmani. Follow him and play the Song of Healing when he asks you to heal his soul.
Go in the water in Great Bay and push Mikau out of the water. It might not look like he is moving though since Link doesn't go in a pushing position. Play the Song of Healing.
Part of the Stone Tower Temple.
Fierce Deity MaskEdit
Get every mask, give them all to the moon children with boss masks, then talk to the one with Majora's Mask.
It's part of the Anju and Kafei quest. Instead of giving the mail to Madame Aroma, the fat lady, give it to the Postman. Note that you will have to do this quest twice to get a prize from both of them.
It's part of the Anju and Kafei quest. You have to complete the whole quest and wait with Anju for Kafei and her to get married. Use it in the Mayor's Office to get a Piece of Heart.
Mask of ScentsEdit
Beat the Deku Shrine to the left of the Deku Palace.
Great Fairy's MaskEdit
Go to the Great Fairy in Clock Town as a human.
Mask of TruthEdit
Beat the Swamp Spider House. This is used to talk to Gossip Stones as well as read dog's minds.
Beat the Gorman brothers in a race. This is used to enter Ikana.
Beat Skull Keeta, the giant skeleton, in a battle. He is sleeping under a bridge. This is used to command Stalchilds as well as make Igos du Ikana laugh at you.
You will get it after playing the Song of Storms for the poe that tries to kill you and entering the house that played the song.
Don Gero's MaskEdit
In the Goron home thing, you have to light all of the torches. Use the Goron Mask to break the pots on the roof that started to spin. One of them contains something you have to give to the Goron in Mountain Village. Use it to get all of the frogs in Termina and tell them to meat you in the Mountain. This will include Gecko, but after defeating him and talking to him. The two Gecko's are two different frogs.
Circus Leader's MaskEdit
In the Milk Bar, play a song for the Zora using the four different instruments. This is apparently the Ballad of the Wind Fish and makes Gorman give you the mask. It serves no purpose except getting the Fierce Deity Mask. It also maskes the Gorman Brother's ignore attack Cremia's Carriage, but you had to do that quest to get this mask. I guess you can do that quest again if you want a hug from Cremia.
Beat the Ocean Spider House on the first day or night to get the Giant's Wallet and then buy it from the Curiosity Shop for 500 rupees. Use it to stay awake for the old woman's stories.
Talk to Guru-Guru, the creepy music box guy, and listen to his story. He will have to be sitting in the Laundry Pool area for it to work I think. Use it to get the Bunny Hood and to make Igos du Ikana's minions do a hilarious march that makes them easier
Use the Bremen Mask on the chicks and collect all 10 of them without stopping the march. They will turn into regular Cucoos and make the creepy mohawk guy give you a this mask.
If you watched Jayle's videos, you would see that guy take the old lady's luggage. You have to stop the guy, Sakon, using your sword.
Go to the rock where he is dancing, talk to him, then play the Song of Healing. It is used to dance with the Rosa Sisters.
Go to the Mayor's Office and talk to the fat lady in the other room to the right. She will give you this mask. This is used in the Anju and Kafei quest.
This is part of the Anju and Kafei quest. You have to go to Kafei's house when his mail is delivered
Beat the ghost poes for Romani and come to the ranch in the evening and catch a ride with Cremia. Protect the milk from the bandits and she will reward you with this mask. It lets you enter the Milk Bar.
Go to the opening Ikana area and use the Lens of Truth where you see a circle of rocks. There is an invisible guy that you have to give red potion to. He will reward you with the Stone Mask. This makes you invisible to most enemies.