In order to run Jekyll Now locally, so I can see changes before they are committed and published publicly, I’ve needed to get Ruby installed. However each attempt produced this error

ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.

This answer on StackExchange which advises using this automated script (laptop), in combination with ensuring that the correct instance of ruby is in the PATH variable solved my problems.

The automated script can be run multiple times. In fact you should regularly run it to keep the components up to date. Thank you Moncef.