Keaton Mask: This is the first mask you can borrow. Go to Kakariko Village and up to the gate to Death Mountain, and talk to the guard. He'll buy the mask from you for fifteen rupees, then go back to the shop and pay the shopkeeper fifteen rupees. Then you can borrow the Skull Mask.
Skull Mask: After you get this mask, enter the Lost Woods from Kokiri Forest and go left. Put on the mask and stand on the stump and talk to the Skull Kid. He'll come and buy it from you, but not the entire price, though! Then go back to the shop and pay the rupees and borrow the Spooky Mask.
Spooky Mask: During the day, go to the Kakariko Graveyard and show the wandering kid the mask. He'll buy it from you for thirty rupees, this time the entire price! Then, go pay back the rupees at the shop and borrow the Bunny Hood.
Bunny Hood: This one will be a challenge of selling it because you have to sell it to the running man in Hyrule Field. Go by the path east of the entrance to Lon Lon Ranch and you should see the man running along late in the day. Follow him until he sits down, and then show him the mask! He'll buy it for so much money it's unbelievable and you only have to pay the shop fifty rupees of it! Go to the shop and pay it off, so you can finally borrow the Mask of Truth, which lets you talk to those odd-looking stones.
You can also
borrow some other funny masks, like the Gerudo Mask, Goron Mask, and
Zora Mask! Go to those people with the mask on and they'll say
different things to you. It's not important to your quest, but it's fun!