The Happy Mask Shop in Hyrule Town Castle Market allows you to borrow a mask to try to sell it to an unsuspecting resident of Hyrule. Just wear the mask when talking to the right person and they'll ask to buy it.
The first mask, which you may have sold already, was the Keaton mask, which goes to the guard who let you access Death Mountain from Kakariko Village.
The second mask was the skull mask. Before selling that one you should have gone to the area of the Lost Woods with the two business scrubs and jumped down a hole in the grass near some butterflies, and showed the mask to the deku scrubs in here to be able to hold more deku sticks. (This area is referred to in the game as the Forest Stage, by the way.) Then you should have sold it to the skull kid standing on a log near the entrance to the Lost Woods (and lost money because he's a cheapskate).
I think that's where I left you off, so now the spooky mask should be there to borrow. It goes to the cute little kid who inexplicably likes to hang out at the Kakariko graveyard.
After that you get the bunny hood. The guy who wants this can be hard to find. When you exit Hyrule Town Castle Market into Hyrule Field, turn it to night if it isn't already and make a right from the castle gate. A guy will be somewhere along the path, and he might be running slowly or just sitting there. If it's daytime he'll be running like crazy and you won't be able to catch him. Once you find him and show him the mask, he pays you a ridiculous amount of rupees, filling your wallet completely. That's the last mask.
Now when you go back to the Happy Mask Shop there will be four new masks. The first is the Mask of Truth, and you may recognize the design from those weird stones around Hyrule that tell you the time if you hit them (they do other weird things if you use other items on them, like bombs or Din's Fire). If you wear the mask of truth and check on one of those, the stone (gossip stones, they're called) will tell you a little bit of information. Sometimes it's useless, sometimes it's interesting. The mask of truth is not necessary to kill the game but it can be fun to have.
The other three masks resemble the different races of people living around Hyrule: Gorons, Zoras, and Gerudos. To get any of these masks you have to trade in the mask of truth. Wearing any mask (including the old ones you sold) can make people say different things to you. Wearing the Goron mask in front of a Goron, for example, might prove to be interesting. Or boring. But play around with the masks if you feel like it.
Since we don't want to have to return to the past again (I don't, anyway), let's go around Hyrule collecting the remaining gold skulltulas that can only be obtained as a kid.
By the moat of Hyrule Castle is a tree, and a hole will open near it if you play the Song of Storms. Make your way to the tree, play the song, and drop down below. Bomb the three brown walls here; behind one of them is a gold skulltula.
You may have missed a gold skulltula near the entrance to Gerudo Valley (if your map shows a gold skulltula when you highlight Hyrule Field, you got it). Bomb the center of the circle of rocks to reveal a hole, drop in, and use Din's Fire to get rid of the webs to get the gold skulltula.
Another gold skulltula in Hyrule Field is reached by bombing a tree to the right of the Hyrule Castle Town Market drawbridge, and jumping down the hole.
The bean holes are only there when you're a kid, so you have to get the gold skulltulas from them now. There were a couple bean holes you couldn't access before. One of them is at the Desert Colossus but I'll leave that until later when we're ready to continue the game. The other one you may not have gone to yet is in Death Mountain Crater, you have to play the Bolero of Fire to reach it, so be sure before you go that you have a bug in a bottle as well as a magic bean to plant there.
Bean Hole Locations:
- Lake Hylia (next to lakeside laboratory)
- Desert Colossus (on the ledge of the temple entrance)
- Gerudo Valley (down below the bridge by the river)
- Death Mountain Trail (in the entrance to Dodongo's Cavern)
- Death Mountain Crater (only accessibly by playing the Bolero of Fire as a kid)
- Kakariko Graveyard
- Zora's River (no skulltula comes out of this one)
- Lost Woods (near where the skull kid hangs out)
- Lost Woods (where the two business scrubs are spitting nuts)
- Kokiri Forest (next to the shop)
Pieces of Heart
Now that you have the Lens of Truth, you can easily win the treasure chest game in Hyrule Castle Town Market (open only at night). Just use the Lens of Truth to see through the chests, to find which one holds a key. (You can try to do it without the Lens of Truth if you want to try your luck.) In the last chest is a piece of heart.
You can find another piece of heart by bombing a tree by a creek to the west of Hyrule Castle Town Market. You will have to have the Golden Scale (as a child) or the Iron Boots (as an adult) to sink below and reach the piece of heart.
Go now to Zora's River and find the music-loving frogs. Play the Song of Storms for them to get a piece of heart.
Make the frogs come back up again by taking out your ocarina, and play their bug-catching game to win another piece of heart (press the correct button to make the correct frog jump up when a bug is above it).
Warp to Lake Hylia and go into the Lakeside Laboratory. Using the Golden Scale you got at the fishing pond, dive down to the very bottom of the water in the lab. Once you've hit the bottom, you'll hear the Zelda "discovery" tune. After surfacing, talk to the creepy guy in the lab for a piece of heart.
Go to Gerudo Valley and grab the Cucco, then hop off the left edge of the cliff with it. There is a ledge on the far cliff with a crate on it, inside of which is a piece of heart.
With a magic bean and a bottled bug in tow, play the Requiem of Spirit to get to the Desert Colossus. Just outside the temple entrance, there's a bean hole on the ledge. Drop in a bug and plant a magic bean, and kill the gold skulltula that comes out.
Switch your shield to the Hylian Shield if it isn't equipped already. Now you're ready to go into the temple.