I started with an iMac. no Sata Cable to add second drive. The mac had a 256 SSD. I swapped that for a 512 Samsung SSD. I also added the SATA cable and the Hard drive secondary doc for a 1TB drive.
It was a pretty easy repair. All the time I spent was making sure I kept my hands off of internal parts and being careful with connectors.
Be careful. Keep your screws in order as always. I think that is always most important.