I highly recommend playing through this game, doing everything in the more or less "correct order". After all, it has an amazing story and a whole mansion to explore!
That said, if you just want to get this achievement quickly, follow these five steps:
1.
*** Spoiler - click to reveal ***Go to the cupboard on the left. Move the duck away, and pick up the key that was under it. Now you can unlock the front door and go inside.
2.
*** Spoiler - click to reveal ***See the wide set of stairs in front of you? To the right of the stairs are some bookshelves. (One of the shelves has a painted skull sandwiched between two trophies.) Between the bookshelf and the staircase is a tiny bit of wall. Move up to it, and click it, then immediately take a step back: You just opened a secret door!
3.
4.
*** Spoiler - click to reveal ***Now go up the stairs. You'll find yourself at the start of a hallway. Follow its horseshoe shape until you reach some glowing, red Christmas lights and a warning sign that says it's Sam's dark room. Click on the attic entry in the ceiling, and the ladder will drop down and extend. You'll lose control for about a second or two.
5.
*** Spoiler - click to reveal ***Go up to the attic, and proceed as far inside as you can. You'll end up in front of a green folder with a desk lamp shining on it. Click on the folder, let the credits roll(or skip them), and the achievement will unlock as soon as you're back to the main menu!
But seriously, play through the game properly! It's worth it.