The following steps allow Link to get the Mask of Truth.  Although it is not crucial to finishing the game, I thought I would put it in any way.

1) The egg.
Get it from Malon just outside Hyrule Castle.

2) The Chicken.
Wait for a day for the egg to hatch.

3) The Letter.
Use the chicken to wake up Talon by the castle. Then go inside the castle past the guards and find Princess Zelda.  She'll give you the letter.

4) The keaton mask.
Show the letter to the guard guarding Death Mountain in Kakariko Village.  He'll ask you to get him a mask.  Go to the Happy Mask Shop in the Market to borrow the mask.

5) The skull mask.
Give the keaton mask to the guard in Kakariko village.  Then go back to the Happy Mask Shop, pay him the money, and get the skull mask.

6) The spooky mask.
Go to the Lost woods. Go left to find the Skull kid.  Stand on the short stump wearing the skull mask. (Note that first you need to play Saria's Song for him if you have not done so already). He'll buy it from you.  Go back to the Happy Mask Shop and pay them the money and get the spooky mask.

7) The bunny hood
Go to Kakariko Village during the day time and talk to the kid in the Kakariko Graveyard wearing the spooky mask.  He'll buy it from you.  Go back to the Happy Mask Shop and pay the money,
then get the bunny hood. 

8) The Mask of Truth
Run around Hyrule field and look for a man running around the field.  He doesn't appear until after you've beat Jabu-Jabu's belly.  To find him quickly, as soon as you exit the Market run toward the right side of Lon Lon Ranch and then circle around it a few times and you should soon see him in the distance. When you find him, follow him around until he becomes tired and sits down (he won't talk to you until he does).  He'll get tired quicker if you play the Sun Song and make it dark then wait for a little bit.  Wear the bunny hood and talk to him and he'll pay you a ton of money for it.  Go back to the Happy Mask Shop and pay them the money and then you can borrow the Mask of Truth! 

9 - Other masks
You can also borrow a Goron mask, a Gerudo mask, and a Zora mask now.  Try wearing them in different places and see the responses people give you.