Step 1: Gut It.
You want to remove everything, but very carefully. There are a lot of re-usable components here, including creative decorations for some of your other projects. We want to preserve the floppy drive door action mechanism as much as possible. Luckily, that's easy. But unfortunately, I didn't photograph it and can't find a suitable image on the net. You will find it in successive steps, though already attached.
Step 2: Protect the HDD Electronics.
Step 3: Connect It Up.
The kit comes with a molex Y-adapter cable so that you can provide power to the adapter and your IDE hard drive from the same source. Attach the molex power cable and the USB cable to the adapter. Then attach the IDE extension cable to the adapter. Finally, attach the IDE cable and other end of the molex Y-adapter power cable to your hard drive. If you're using the same adapter kit it should end up looking like the last photo.
Step 4: Insert Into Case.
The Disk II has a nice cable-management clip which used to be for the colorful original ribbon cable. Use this for stress relief on the USB cable. I wasn't able to angle mine to use it for the power cable as well. But as it turns out, we have a nice fit with the top on, without smashing cables.
Step 5: Re-attach the Floppy Door.
When the door is closed, it should not be pressing down at all on the adapter+HDD combo. Mine just fit so perfectly that it prevents the IDE extension cable from lifting the USB adapter.
Notice the ridiculously simple door mechanics. Also notice how the door lever is eating up a lot of important space, right in the middle. If you didn't care about using the door, you could detach the plastic part of the door using the two screws on the right side, and then glue it in place. You would then have loads of extra space--probably enough for a 2nd hard drive.
I have a plan for the door, though, so I'm leaving it alone.
Step 6: Close It Up.
Make sure both the power and USB cables exit through the little notch at the back, as in the photo. The cable on the left is for power, and has an integrated switch. That's what I'm using to switch power to the disk and adapter until I re-mod the case to use the door as a switch (see last step for the plan).
Step 7: Test It.
Step 8: Future Mod Possibilities.
You can insert a knife switch near the front of the door, though you may have to get creative to find room for it. This can be used to control power or a small LED light show circuit.
You can substitute a 2.5" laptop drive for the 3.5" drive. Then you'd have plenty of space for the knife switch. In fact, it might then obviate the need for the IDE/SATA extension cable we used in step 3. (I haven't verified that it will fit in-line, so caveat lector.)
You can wire up the original "in use" activity LED to blink when the HDD is in use. Here's an example site explains in detail how that can be done:
Finally, you can add another molex Y-connector and a fan, to keep the interior cooler.