This is actually a rather easy achievement to unlock, even if it sounds daunting. It might take you a couple of tries, but shouldn't really take more than that.
First off(depending on your computer), you might want to set the graphics settings as low as possible, as this can affect the speed of the game, and possibly make you lose a second or five. I also suggest doing this after having played through the game at least once before.
With that said, let's begin!
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1. Start a new game, and don't enable any modifiers. The loading screen will show - at this point, spam the Escape button to skip the intro and avoid wasting any time there.
2. Once in control, go immediately to the left and open the cupboard, remove the duck, and pick up the key. Go then straight through the door.
3. Go to and open the secret passage directly to the right of the staircase. You'll have to get up close to the wall, click, and then take a step back so the door doesn't get stuck on you.
4. Go inside and pick up the key there, then immediately turn around and move outside and up the stairs.
5. Turn and walk towards Sam's room. Forget about turning on the lights. Turn 90 degrees to the left and walk until you either see the door in front of you(it'll sort of light up slightly) or the red to your left. Turn 90 degrees left again, and you'll see the red Christmas lights.
6. Go towards the red lights and click on the entry to the attic in the ceiling. You'll lose control for a couple of seconds while the ladder drops and extends. Just keep pushing forward so you'll immediately move as soon as you regain control.
7. Go up to the attic, then just straight through and towards the green folder at the end. Click on it, and you'll probably unlock the achievement!
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It might take a couple of tries to avoid getting stuck and navigating everything perfectly, but it's much easier than it sounds. So if you didn't get the achievement immediately, chances are you were just a second away from getting it, so just try a couple more times and you'll probably have it very quickly.
Just remember: Duck key, go inside, secret room key, go upstairs, horseshoe hallway, go to attic, go to the end. Easy peasy. Just keep moving.
Oh, and it doesn't matter if you watch the credits or skip them.
Good luck!