There's two ways to go about the situation:- the "Mask Collector's Way" and then there's the "I-Can't-Be-Bothered-Collecting-All-The-Masks-So-I-Won't-Be-Able-To-Get-The-Ultimate-Transformation-Fierce-Deity's-Mask Way". If you have all 23 masks you should talk to the kids who are wearing the Guardians' Masks. Each one will ask you for masks. You must give them one mask for whatever number temple Guardian Mask they are wearing (eg. Twinmold Child wants 4 masks, Odolwa Child wants 2 etc.) so hand over the masks. You'll be teleported to a mini-dungeon, which will fit in with the theme of the Guardian Mask's temple (eg. Gyorg Child's mini-dungeon involves negotiating a series of high-speed currents and Odolwa Child's mini-dungeon involves Deku Flower jumping). Once you've found the kid at the end of the mini-dungeon, you'll have to give them the same number of masks you gave them to get in.
Once all the kids that were running around the tree are gone, talk to the one leaning against the tree, he's wearing Majora's Mask. He'll hand over the Fierce Deity's Mask and challenge you to a game of good guys and bad guys, you're the bad guy. Then you'll be teleported to a large and trippy room, smash the pots to either side of you and collect what falls out. If you have the Fierce Deity's Mask, then put it on now. With this mask on you can just beat the life out of the mask until it kicks the bucket, but if you don't have it, or are up for a challenge, then strap on your Bunny Hood (seriously) and follow the guide below. Whatever you choose to do you must first take a few steps forward to initiate the final battle.