I must be more of a beginner than I thought I was! Is there a set of step-by-step instructions for converting a MacBook ’09?
Converting as in installing your preferred version of Linux?
Yes. I was thinking of Mint. ??
Mint is my daily driver, I REALLY like it because it just gets out of the way and lets me do my thing. The cinnamon UI is pretty nice, there are 2 other options which are somewhat more ‘light weight’, so depending on the configuration of your mac one of those may be a better choice.
You’ll need to create a bootable USB device / thumb drive, using whatever program is available to you. Rufus or Balena Etcher on Windows, Balena Etcher or whatever else on Mac, Balena Etcher and some others on Linux.
Once that’s done, plug it into the mac & power on, hold down the option key & you SHOULD be presented with a selection of items to boot from - as in the Mac’s internal disk and the USB device. Select the USB and let 'er rip! 
This worked on a pair of 2013 macbook air’s, not sure if the power on / key press sequence is the same for your 2009 machine.
Wow! Thank you for the help! So great to know where and how to start!