What are the reverse steps for installing the dual drive kit?

All the iFixit install/replace guides are the same: Follow the steps to perform the disassembly/install, then follow the same steps in reverse order to reassemble the machine.

My own practice when I'm using an iFixit guide is to print out the guide (in color, if my color printer is working that day), and tape the removed screws/components to the pages at the place where they're removed. That way, I have the correct parts in the place in the guide where I'll need to reinstall them.

I was under the impression that Apple was using two drive bays in Fusion Drive installations: One for a standard 2.5" HD and one for an SSD. The Fusion Drive is then configured in software, through a tweaked version of Disk Utility to merge the two drives together into a single user-visible volume. Have you already taken the mini apart? Are you sure there's an empty space for a second drive? I would think that a Fusion drive installation would already have a dual-drive bracket in place, but both bays would be filled. Unless you're planning on converting to two SSDs, installing two drives would give you more storage space while losing speed.

My bigger concerns here are that:

• The Fusion volume, although it uses two physical drives, is a single volume. If you remove one device from the set, all the data currently on the drive will be unreadable. If you separate the hard drive from the Fusion array, you'll have to reformat it in order to use it as a standard hard drive. If you want to keep your current data, you'll need to get it onto an external drive first - either using Time Machine, or cloning it using Disk Utility, Carbon Copy Cloner or SuperDuper!.
• You may have bought a hard drive mounting bracket you already have pre-installed.
• If your plan is to install an SSD in the normal boot drive slot, and a standard hard drive in the second slot, you already have that. If your plan is to install two standard hard drives, you'll gain space and lose speed. If your plan is to install two SSDs, there's a long history of quirky behavior from SSDs installed in the second slot of iMacs, laptops and minis.

What's your plan here? What are you trying to gain through this installation? If all you're looking for is more storage space, you might be better off with an external drive (Thunderbolt, FireWire or USB3). Since it's a desktop system, you can get a big 3.5" desktop drive, which are larger/faster/cheaper than most 2.5" HDs. If you're using an external drive, you can expand your storage just by attaching another drive when you need one.

Note: All these concerns apply only if you already have a Fusion Drive. Is that true? I suspect you can find out in several ways; Disk Utility should tell you whether a volume is Fusion-formatted, and the SATA section of System Profiler should identify the separate SSD and HD mechanisms, if you have them.

On the other hand, if you have a single drive mechanism and it's a Fusion Drive, I think a lot of us will be very interested to hear about it.