I was having system overload errors using Logic, and both Disk Warrior reported that my HDD was failing.
Very smooth! I took my time and it was much easier than I expected. The hardest part was getting all the little plugs out and routing the new wires. It is fun to look inside the machine! I added a 256 Gb SSD from Monoprice and put in a 1 Tb Western Digital HDD from an old external that I had, which looks like it is OK.
Once I got it back together formatting the new drive was straightforward. I did a new install of OS X Sierra and then used migration assistant to get my apps on the SSD. My confusion now is about the data. Migration assistant makes a new backup and I cannot see how to use it to get the data. I ended up dragging my home folder over to a new folder on the HDD, and then targeting that in user accounts. I could not find a lot about setting up the drives to keep system and apps on SSD and data on HDD. Here are two links with suggestions. https://www.youtube.com/watch?v=Ou-heXnk...
I am open to any other advice - I would like to have my system library and maybe desktop on the SSD.
The machine is very fast now! Should have done it long ago. Thanks ifixit! Sorry no photos.