Backup any data you have on the original before you start doing anything.
Make sure you know how to take static electricity precautions, and use the right screwdrivers as documented on http://www.ifixit.com/
The cable you see is usually unnecessary. Just make sure not to dismount the bracket that is attached to the chasis of the computer or unscrew it, as it is attached to the heat sensor cable which is very fragile and easy to damage. That cable is only if you damage it in the process. Usually removing the drive can be done simply by removing the top part of the inside bracket and carefully sliding it out and detaching the SATA cable. Generally it is unnecessary to do anything with the IO cable, as that is very fragile and easy to damage trying to reconnect it to the logicboard.
Use the command-option-shift-R internet restore while hooked up via ethernet after inserting and connecting the SSD, to use Disk Utility to reformat the WD drive before installing any OS. Use the Migration Assistant after upgrading the internal drive to match the OS on the old platter drive, and connect the old platter drive via a USB-SATA drive reader, that has its own AC power.
Take it from an Apple Certified Tech, if you take the precautions I indicate to the letter, you should be fine. If you have any doubt, ask an Apple Certified Tech to do it for you.
