WP Toolkit is available to be installed in the WHM Marketplace interface.
Access the WHM Marketplace by logging in to your WHM as root and going to "Server Configuration / WHM Marketplace." On this page, access the "Add Extensions" button in the top right.
In this list of available Extensions, you will find "WP Toolkit" and the option to install this package.
Pressing the "Install" button will install the WP Toolkit on your server.
WP Toolkit can also be installed via the command line as explained here: How do I install WP Toolkit via the command line?