To install "festival" you need to be at a terminal prompt on the Raspberry PI. Then type:
$ sudo apt-get install festival festival-freebsoft-utils
You will get asked to confirm the install. Type "Y".
To make the PI talk, just issue a command from the terminal prompt, such as:
$ echo "Whiskey Tango Hotel dot com Where stupidity meets reality"| festival --tts
That's it. Of course, you can also execute this from a Python script, etc.
Here's a sample of the output. Sounds pretty good to me.