My mid-2012 15" MacBook Pro has been getting ever slower. My last two work laptops came with SSDs, but this older machine has a 768GB hard disk drive. It was time to get a solid-state drive, which are far more affordable now than four years ago. Rather than replace the HDD, I bought a Data Doubler kit and a 480GB SSD. I removed the optical drive and installed the Data Doubler with SSD in that space. I rarely used the DVD drive, so I won’t miss it.
The Data Doubler kit came with a set of screwdrivers and a detailed manual that covered multiple MacBook models. The manual had combined instructions for both the 2011 and 2012 15" MacBook Pros, which weren’t quite right for the mid-2012 model. The iFixIt instructions cleared up my confusion regarding the camera cable connector. That aside, it went flawlessly.
I used Disk Utility to format the new drive as HFS+. I had previously made a bootable USB thumbdrive of OS X 10.11. I booted from the thumbdrive by holding down Option ⌥ as the laptop turned on. I installed a clean copy of OS X on the new SSD drive, leaving the original HDD untouched.
I’m still setting up the new installation, but the laptop does seem much more responsive than before.