A second computer (again, may not be needed but also might be essential for troubleshooting).A macOS installer for your chosen version of macOS (can be downloaded from the App Store).An extra flash drive (may not be needed but can be very useful).A linux ISO or install disk (Any should work but I picked Ubuntu Desktop).A Mac running a recent-ish version of macOS.Full disk encryption for macOS using filevault.
Doing it this way meant that I had no data on the SSD while I experimented with partitions and boot managers although everything I did should be possible on a drive with an existing system. I decided that I wanted to make my system faster by replacing the hard drive with an ssd and while I was at it I decided I also wanted to have Windows and Ubuntu partitions. I have a mid-2012 15” non-Retina Mac book pro with a 1TB hard drive. That being said this should leave all of your data untouched. Make sure you have adequate (and verified working) backups before you proceed. Disclaimer: This has the potential to destroy all the data on your drive.
