Move! That! Bus! Achievement in Goat Simulator 3

Before you can unlock this achievement, you'll first need to level up your Goat Castle to S tier, as this unlocks all areas and decor in the castle. Refer to New Goat Order (15G) for doing so.

Once you've got it all unlocked, you'll need to go through and change the decor at every possible place inside. Starting from when you enter the castle through a Goat Tower, here is a list of all of the places to change decor. You need to press  when prompted and then scroll left or right at least once to change the decor. Just do it once so it's easy to be sure you changed it from the default decor.
 

1. Immediately when you enter, a statue on each side, left and right, for two statues total. 
2. In the main open area, turn around as soon as you walk through the doorway from the above area. There are two little statues on both sides of the door you entered through, and two huge statues just beyond each of those, farther left and right. 
3. Left (if your back is to the entrance) where all the portals are, there are eight trees in a ring around the area. 
4. Up the steps from there (just back in the main hall) is a giant statue at the top. 
5. Head in the doors on the left (after contining toward the throne) and there is a pile of poop on the ground, before you get to the tentacles in the hall. It'll be tough to spot, so just look for the prompt to press  to change decor
6. There are two trees, one on each side of the dance floor, looking out over the main floor. 
7. There are two statues, one on each side of the steps leading up to the throne
8. There are two statues, one of each side of the door to the boss fight (the one you removed the chains from)
9. There are three paintings on the steps leading up to the big light switch. You can reach each from the steps by the railing.