First things first, finish the game at least once, so you'll know the house layout and key locations.

You can do this glitch right after seeing the clown standing near the sewer entrance, but doing so will make obtaining the key inside the office's briefcase impossible, so get that first.



Now walk back to your bedroom and open the window and the lower desk drawer to your right.



As you can already guess, you can walk on the drawer and then outside the window, reaching the ledge above the garden. If you walk to your right you'll be able to see what's behind that mysterious locked door you can't open in the game: nothing :(



Now, without leaving that ledge, walk all the way to your left until you hear the clown's laughter. Wait a little and you'll be able to see him running in circles below you.



You're safe on the upper floor, just don't go down in any way because he'll kill you on the spot.
What you've to do now is walk back into your bedroom (is kinda tricky, try to walk close the window frame), then go into the bathroom and click on the curtain's cord on the right of the window (don't open it) and walk back outside the bathroom ASAP because he'll get you if you don't.



That's it. He'll stuck there for the rest of the game and you'll be free to roam the other side of the street, check safely the corridor next the treehouse and enjoy the glitched ending.