First, you must be young link and already have bombs, the boomerang, and the Song of Storms. Then go to hyrule castle, sneak past the guards, and go by the tree next to entrance of the castle(it's past the moat). Play the Song of Storms and a hole will open appear on the ground. Go into the hole and you'll see three bombable walls. One of these has a gold skulltula inside. You must kill the Gold Skulltula, then stand in front of the circle pad that transports you back up. Now, throw your Boomerang at the token and as soon as it hits the token step back on the pad(before the boomerang comes back to you). You will be transported back to the surface with a new Gold Skulltula token. The Gold Skulltula should reappear down in the hole if you did this right, so now you can easily get 100 tokens!