Open the Github Depository at https://github.com/ohnosec

We are interested in the ZXPrinterLibary and ZXPrinterTester.  We will download them in order.  First click on ZXPrinterLibrary

Click the CODE button and then in the drop-down menu select "Download ZIP"
save this file in a location you can find
Then navigate back to the https://github.com/ohnosec screen and follow the same procedure to download the ZXPrinterTester Library

Click the CODE button and then 
in the drop-down menu select "Download ZIP"
save this file in a location you can find
Then navigate back to the https://github.com/ohnosec screen and follow the same procedure to download the ZXPrinterTester Library

You should end up with the two libraries in a location you know.

Open you Arduino IDE.  It does not matter which sketch you have open.

Then in the menu navigate to and click on
Sketch > Include Library > .Add ZIP Library...

In your file browser navigate to and click on the ZXPrinter-Library-main.zip file that you downloaded from Github 

You should get a message in your IDE saying "Library Installed"

One way to verify that the ZXPrinterLibrary installed is the following.  In the menu goto File > Examples > Examples from Custom Libraries > ZXPrinterLibrary and see the 5 examples are now available

The ZXPS libraries require the digitalwritefast library. To install it or confirm that you have it...
Open the menu Tools > Manage Libraries and type in "digitalwritefast".
If it isn't installed it will give you the option to install it.  Do so.