Before installing ponysay, make sure your system have the packages listed under Required runtime dependencies and Package building dependencies installed.
Tarballs can be downloaded at https://github.com/erkin/ponysay/tarball/master for bleeding edge, or from https://github.com/erkin/ponysay/tags for releases.
If you have git you can clone the project URL https://github.com/erkin/ponysay.git.
In the terminal, cd into the ponysay directory and execute ./setup.py install or python3 setup.py install. This will install ponysay into /usr, normally meaning you need to run as root, e.g. by running sudo ./setup.py install.
Now you will be to use ponysay, run: ponysay "I am just the cutest pony!", or if have a specific pony in your mind: ponysay -f pinkie "Partay!~".
ponysay comes with this info manual and a manpage in section 6, man 6 ponysay (or just man ponysay). The manpage is also available in Spanish: man -L es 6 ponysay. To install the Spanish manual add the option --with-man-es when running ./setup.py.