Download and unzip the file, then run the checker script to start the application.
But not all of us who use Linux these days are quite as geeky as the programmer thinks. So, step by step, here's what will work (based on my experience):
1. Go to the Checker page - https://cgjennings.ca/checker.html
2. Click the 'Download for Linux' link
3. Save the compressed file - checker-unix-0.96.tar.gz - to [whatever folder you choose for such Downloads]
3. Extract checker-unix-0.96.tar.gz - you can probably just right click on the file and choose "Extract".
4. The "checker" script that you have to run has been extracted in a folder of the same name along with other files.
5. Open the "checker" script in a text editor and follow the programmer's instructions -
"Uncomment the following line ....".
(That is, just edit out the # at the beginning of the two lines that begin "INSTALL4J")
5. Open a terminal and as root (su or sudo) give yourself permission to run 'checker' by typing chmod 777 path-to--checker . (I chose the easy option of giving everybody permission to do anything by specifying '777' ).
6. Now - Run checker! You do this by navigating to the folder containing the 'checker' script, opening a command line and typing: ./checker
7. If, like mine, your Java is a recent version (11 here), running the checker script will give you the message that:
The version of the JVM must be at least 1.6 and at most 1.8.
Please define INSTALL4J_JAVA_HOME to point to a suitable JVM
In which case, you can google "openjdk-8-jre", download and install it with your package manager,and you will get an earlier JVM (Java Virtual Machine) which will work with Checker. I uninstalled my openjdk-11-jre package first.
And now you run checker again and all is well!
8. For anybody using the KDE Plasma Desktop Environment, you can add Checker to the application launcher and give it a nice icon:
Right click on Kickoff.
Mark the folder you want it in, e.g. Utilities. Click on New.
Fill in the name you want.
Add the actual command and click on the square next to name to change the icon.
Hope that saves at least one other Librivoxer some time.
Disclaimer: I'm not a programmer or even a proper geek.