This sequence is also shown in a short video walkthrough.
Overview: The screenshot above shows the door to the basement, where the Racetrack Key is hidden. Since Lara doesn't like anything too simple, the switch that opens this door is way up in the attic. There are also a few obstacles in the way. Here are step-by-step instructions for getting the key.
Press the button next to the attic door to open it. (You can explore up there now if you like, but it's not necessary. You'll be coming through a different way shortly.)
Continue along the mezzanine to the MUSIC ROOM. Go through it and to the left into the library.
Find the loose book in the bookcase on the right and pull it to extinguish the fire in the fireplace.
Step into the fireplace and scale the climbable wall on the left.
At the top, climb to the right and then let go to land on the ledge.
Follow the secret passageway to a room with a switch and a couple of wooden crates.
ADJUSTING BRIGHTNESS/GAMMA: The default brightness setting in this game is fairly dark. To conserve flares in dark areas like the ATTIC, you can adjust the gamma setting in the PC/Mac version. Choose the Video options (sunglasses) from the main menu and set the gamma higher. For the PlayStation version, you'll probably need to turn up the brightness on your TV.
Pull the movable crate on the floor once.
To move a box or block, position Lara with her chest against it. Press and hold Action. Lara will grab the block and go into a slight crouch, indicating that she's ready. While holding Action, press Forward or Back to push/pull the object. Keep in mind that there must be sufficient space behind or in front of the block for Lara to move it.
Behind it are some more flares.
Climb onto the ledge and pull the second crate twice (through the doorway and into the room). Lara will end up standing on the first crate.