It's quite simple. If you search the forum for "optibay" you'll find dozens of threads with instructions, prices, recommendations, etc. that you'll likely find useful.
If you can install an HDD in your iMac (Alu?), then installing the optibay is much more easy. I did both and installing the optibay is only a quarter of the work of installing an HDD in an iMac.
With a good work surface, enough light, some patience, a Philips 00 screwdriver, a small magnet, a pry tool, and an installation video, it is pretty simple.