Happy Mask side quest

The Happy Mask side quest begins when link shows the Hylian knight Zelda's letter allowing him to enter Death Mountain in the Legend of Zelda: Ocarina of Time. He asks Link to bring him a popular mask for his kid. At the end of the side quest you gain the mask of truth which allows you to listen to the Gossip Stones.

Keaton Mask
Head to Castle Town and enter the Happy Mask Shop. Inside speak to the mask salesman and barrow the only mask availiable. Head back to the knight and talk to him while wearing the mask. He will give you a certain amount of rupees to keep and give back to the salesman.

Skull Mask
The Mask Shop now has a skull mask you can barrow after you pay the salesman. head to the lost woods and find the open area with only one skull kid. Wear the mask, stand on the stump, and play Saria's Song. After playing he will ask to have your mask which you give him and he gives you a few rupees. Now head back to the Salesman.

Spooky Mask
Barrow the mask and head to Kakariko during the day. Head straight to the graveyard, wear the mask, and talk to the kid imitating Dampe. He will get scared thinking that you are Dampe. once he realizes that it is not the grave keeper, he will ask for the mask to look more fearsome.

Bunny Hood
Now barrow the Bunny Hood from the Happy Mask Salesman and head to the eastern half of Hyrule field near the entrance of Lon Lon Ranch in the later half of the day. Wear the Bunny Hood and watch for a running man. when he sits to take a rest, talk to him. He will fill up your wallet and head back to the Mask Shop.

Other Masks
You can finally barrow the Mask of Truth which allows you to listen to the Gossip Stones. You can also get the Gerudo Mask, Goron Mask, and the Zora mask.