Adding second HDD alongside SSD, Macbook Pro 13'' (mid 2012)
I didn't find the Macbook superdrive useful any more, and because I installed SSD as the main drive for my laptop some year ago and it was filling up slowly but steadily, I decided to add second hard drive with dual drive kit. The plan was to add a 1TB secondary drive and use it as a storage mainly for pictures and video files, since I bought a GoPro camera, which eats up HDD space like a hog!
Repair itself was quite straight forward. iFixit's step-by-step guide was really good, and I didn't have any problems with the install. Only those small screws begun to loose their base because I accidentally turned them with too much force and with too large screwdriver head. Also I had to go through the whole install process twice, since the 1TB HDD didn't work properly and I had to replace it with another drive. Neither of those was bought from iFixit, though, so it wasn't their fault.
My best advice is just to follow those steps carefully. It's also convenient to place every screw in a safe place and also in same kind of formation as they are in the computer so it is easier to put them back where they belong.
You should also check your additional HDD before you install it and preferably format it as a Mac OS X journaled before installing it, if possible.
Also to prevent any unwanted shocks, I used thin rubber gloves when performing the install. Always remember to ground yourself by touching some metal surfaces other than your expensive computer!