Step 1: Releasing the spring
Open up your stapler.

Most desk staplers have a spring attached to the lil slidey thing that pushes the staples up.

There's usually a lil hook on the slidey thing that the spring slips onto. Ever-so-gently, slip it off. Watch it's path and try to remember it.

Chances are it'll slide right off the hook that attaches it to the body. It should look something like the above pic.

Step 2: Further disassembly

When you push on the long bar thingy, it should move up. While it's up, you can push the other thingy out. While you're holding that thingy out, let the movey thingy slide back down. It should make sense when you do it.

It should look something like that.

Step 3: Removing the staple ejector (optional)

That not-movey thingy from step two may also be removed on some models. For a lot of staplers, it just slides out. It's nice to get it out of your way before you start drilling, but it's not critical. If you can't get it out, don't worry about it.

That's what it looks like when it's gone.

Step 4: Drilling

For plastic, just drill nice and slow. If you can, put something worthless and easy to drill under the stapler. That'll keep the plastic from popping out the other side. You might want to start the drilling using an abrasive bit, then switch to a metal-boring bit, then go back to the abrasive bit. But any bit should have no problem.

For metal, you have to "tap" the surface first. Take a nail and pound it into the metal until you get a small nick. This allows the drill bit to "grab" and drive itself through the metal. If the drill skids accross the surface, you didn't tap it deep enough. When you're drilling be careful to avoid the stuff that gets drilled out, it's pointy and metal. Metal is a lousy thing to dig out of your hand.

It's hard to do this straight. Some people put duct tape over what they're tapping. It provides traction to keep the nail from sliding, and the lines help you go straight. It also cuts down on the dangerous drill-waste. I can't do anything straight, so I can't say how effective it is.
With metal, a metal boring bit is best. An abrasive bit would work, but it'd wear out fast.

When you're done with the drilling, screw in the spikes.


Step 5: Reassembly
Put it back together. You're the one that took it apart; if you can't figure out how to put it back together I sure as hell can't help.

It should end up looking just as nice as this.