I wanted to upgrade a spare 2010 Mac Mini with an SSD boot drive and 1 TB hard drive so it could use it as a server for 13 other computers.
As usual, it was very easy to follow the iFixit guide -- those things really are a godsend! I swapped the existing HDD for the 1 TB drive and put the SSD in the iFixit SATA optical bay enclosure. All done in two hours (apart from earlier cloning the existing drive to the SSD) without any problems at all.
When replacing the power supply in the Mac Mini you might need to take a little time to wiggle it it position -- it doesn't automatically slot back in.
Aside from that, take note when the guide tells you to be gentle -- the SATA connectors and the temperature sensor connectors can take a bit of patience to remove.