Hot Lava

You can unlock a follower by getting all 60 cards in the basement's trial area. There are already guides out there for getting the cards, so I won't list their locations here. Once you have all 60, go to the "Toy Shelf" in the main menu and equip the follower to get the achievement.

By far the hardest achievement in the game. You get stickers randomly from getting stars (completing challenges in different courses). Any star, including the ones from all of the trials, give you a chance to obtain a sticker. There are two sticker sets in the game, but you just need to fill out one of them. As a result of this achievement being heavily based on RNG, you might get either very lucky or very unlucky and have to complete almost every challenge in the game. Note that you are guaranteed to have every sticker if you complete every challenge in the game, but you don't have to complete every challenge in the game for a complete sticker sheet.

The achievement will pop once you trade in a completed sticker sheet.

As of the August 27, 2021 update, the accesory system was overhauled. You still earn accessories with every star earned from completing challenges, but you are now able to trade in duplicates for tickets. The tickets can be used to purchase missing accessories, including stickers. This means that if you get unlucky, you no longer have to complete every single challenge.

The game isn't very clear on which stickers are part of what set, but the achievement unlocked for me upon obtaining the first 32 stickers. Would highly recommend doing this achievement towards the end as you'll have a surplus of duplicate accesories/tickets.