I actually made mine a year or so ago, using another Instructable. Whilst I was able to get by, and I do not wish to insult the intelligence of any Instructables users, I found the original Instructable to be less than helpful at points, and felt it could be made clearer.
Also, you will need:
A Soldering Iron
A spare 3.5mm (audio/headphone) cable (or just plain speaker wire if you plan on connecting this to an amp).
Now I have finally gotten round to posting the process, and writing it up. I hope this helps some of the people who filled up the comments on the original with questions. So without further ado, to the Bat-Workshop!
Step 1: Twinkle Twinkle T-8 Star
These are known as Torx screws. The head has the appearance of a six (6) pointed star. The Wikipedia article explains why they are used, for the curious. Many people will refer to them as Star screws. That's fine.
If you don't already have a set of Torx bits, you should quite easily be able to get some from a hobby/electrical shop or trade store. If you are unable or unwilling to procure some, your job is still possible, but harder (I began the project without them myself).
If you have the tools, get to work taking the screws out. Leave no screw untouched! As they say in retail, everything must go! You're going to want a T-8 bit, most likely. That should suffice for every screw.
If you don't have the tools, you can use a set of pliers or a monkey/plumbers wrench. Even if you have needle-nose pliers though, you're likely to find yourself unable to remove at least one screw, leaving you the choice of drilling it out or buying some Torx bits (I suggest the latter). For now though, simply grip either side of the screw head, and twist!
Step 2: To PCB or Not to PCB..
This is not as easy as it sounds - the magnet is extremely strong, and not only will it happily give you lovely black and blue pinches when you take it out and play with it (be careful not to let it slam back together - the magnet is brittle, and bits break off), it will also tug at your screw-driver, in a most irritating fashion - especially when trying to get the "plier-proof" screw out. It's not easy to actually pull off either.
Unfortunately, even after you wrestle the top magnet free, you will find that there is one final screw to take out, on the other side of the drive. "But all that's left on the other side is the PCB!", says you. Not so, says I. That PCB hides a screw, which holds the other side of the Read/Write Head down. Rip it off the back of the HDD in whatever way pleases you (it is both screwed and stuck on), and remove the screw.
Step 3: Off With Its' Head!
You are now faced with a choice. You could do what I did, and remove the ribbon cable holder and cut the ribbon off as close the Read/Write head as you can (it connects to the Head near two large-ish blobs of solder). Or, you could work out which pair of pins (see image two) lead to traces in the ribbon cable which end up at the blobs of solder, and then solder to those.
Those wishing to experiment have nothing to lose (you can always just cut the cable if it doesn't work). Either way, now is the time for soldering, unless you want to hold the wires in place every time you use this. So, I guess you should go get/borrow the use of a Soldering Iron if you don't have one :D
Step 4: Polarity Irrelevant
See photo two for a detailed imaged of where to solder. Correct soldering technique is to heat up the two parts you wish to solder (using the iron), and then place them together, applying the solder as you do. That never worked out for me.. I'm a bad solderer. Do it however you wish, just try not to solder the contacts together.
If you're using a headphone cable or similar, you're going to have to strip the ends where the headphones attach, so that you can solder them on. If you don't have wire-strippers, scissors and patience are a fine substitute.
So, strip the wire (if necessary) and solder away!
Step 5: The Theory
The Hard Drive generates sound on much the same principle that a speaker does. In a speaker, it is the cone that oscillates, driven by (you guessed it) a voice coil, transmitting sound to our ears by so-called sound waves (GCSE Physics FTW). There is a little more to it than this, but you get the idea.
In the Hard Drive, the voice coil (labelled earlier) moves the Read/Write Head in a similar fashion to the speaker cone. If you look closely, you may see it move. To see an enhanced effect, play a track filled with very low frequency signals (bass to you and me) - the low frequency and long wavelength produce slower, longer movements of the head (although what you're seeing is still 20Hz+ if you can hear it, meaning that the head is not moving as slowly as it appears; See POV).
The Hard Drive Speaker is generally ill-suited to Bass Heavy tracks, lacking sufficent displacement. Higher frequencies tend to come through louder.
Nevertheless it is a fun thing to build and play with. I believe previous Instructables have also recommended attaching wires to the contacts driving the spindle motor (which rotates the platters) in order to provide at least an interesting visual effect. I do not know what effect, if any, this has on sound, but you are welcome to try it!
Ultimately, I hope very much that this is not regarded simply as a duplicate article, although some might see it that way. I also hope that some people find use in it. Thank you for reading.