Text comes from scripts defined by Pastebins. Here's an example:
setBlinkDuration 300 setBlinkDelay 100 setBlinkGroupDelay 1200 setCaptionDuration 2000 setCaptionDelay 1200 bigcap YOU LOVE FLUFFY KITTENS blink KITTENS / ARE / YOUR / LIFE cap Cuddle all the kittens forever because you love them.
In the example above, the following things are true:
Other supported commands:
There are some raw CSS commands, but I don't have the energy to document them right now and I don't really expect people to use them.
The result is dumped into a Pastebin, and the ID of that Pastebin can be put in the "text pastebin ID" field on the main page of the Gooninator.
That should be enough to get you going. Here's a real world NSFW example.