After episode 2 use the sticker book to head back to the Playground area, then in the Playground area head to the right side over the bridge and you will find 3 slides. Go to the very top and you will need to press I think it's E on PC to slide down the slides, but what you need to do for this is spam E until you've completed all 3 slides.
Do not move your directional buttons/sticks/mouse and don't try to change view when sliding otherwise it won't work. Simply stand at the top slide and spam E until you get all 3 slides done. Easy. (I'm not 100% sure on the PC controls so it might not be E button, but it should show you what to press anyway).
If you need help with this you can use my video guide below to show you, it will start at this point: