From the start, head into Guillotine Square for a cutscene. Afterwards, find the lab entryway and into the lab to talk to Dr. Finklestein. While in the lab, examine the bookshelf there for a torn page, which will be used later.
When you leave, head behind the stone walls to a short path leading to the graveyard. Talk to Sally here to get forget-me-not. Take this back to Dr. Finklestein. He'll say you still need surprise.
Head back to the graveyard where you met Sally. Kill the heartless, the examine the coffin in the back of the area to move forward. Here, the mayor will tell you to remember the order of the ghosts on the tombstones & activate them in the same order. Once this is done, the pumpkin will explode & you'll get "Jack-in-the-Box". Take this to Dr. Finklestein.
You'll see another cutscene where Lockem Shock, & Barrel steal the heart you're trying to build. Chase them down by going through the graveyard's coffin, and through another coffin at the area you made the pumpkin explode. Kill all of the heartless at the area with the curled bridge, and you will then be able to interact with a gravestone to make it uncoil to allow you to walk across. At the next area, you can kill the heartless or run straight through to Oogie Boogie's Manor.
In Oogie's manor, first drop all the way to the bottom and get a red trinity thats on a brick archway by the water. This will be protected by a super heartless later, so it's better to get it now. Then, use the walking bathtub to return to the top. Climb up the manor by following the path around. When you get to areas with small platforms you can lock onto, use fira on them to make them rise & fall, then ride them to the next level.
Once you get to the top, you'll have to fight Locke, Shock, & Barrel. I found this fight really easy if you stand on the couch which they're spinning like mad, then run down and hit them when they're dizzy. If you try to stay on the ground, it can be frustrating - especially on proud difficulty. After the mini battle, hit the lever that is in the room so the green door they talk about appears.
Run along the wooden pathway outside until you reach the big metal mouth. Jump down & land on the stone platform. From here, you can follow the path to the green door that takes you to Oogie's Torture Chamber. If the door isn't there, go back up to wher eLocke, Shock, & Barrel are to make sure you didn't miss the lever. Inside, you'll fight Oogie.
Oogie isn't a hard battle. He'll run around a top layer and throw things at you. When you see the tiny buttons near the middle of the platform you're on glow, you need to step on one while you & Oogie are in the same section of the spinning wheel. If you time it right, you'll go up top with Oogie & can hit him a few times. If you get the timing wrong, you'll spin to some statues that shoot at you - just hang to one side or the other to dodge this. Get the timing right 3-4 times to win the fight.
When the cutscene ends, Oogie's manor will be alive. You'll need to jump to the bottom and destroy the 7 purplish orbs all over him to win. The fire coming from Oogie's lantern can do some big damage, so keep your health high as you climb and destroy them. Overall, this isn't a hard fight. Once all 7 have been destroyed, you'll seal the keyhole here. Board your gummy ship for the achievement!
If you're struggling, here's a video guide. I did not make the video, so all credits on it go to the original creator (it's from PS4 because there are more videos available from there, and I suck at video creation. Text guide is by me though!):