How do Finn and Jake know Prismo's ritual?

Question

In the episode 'Is that you?' Finn and Jake sing a song that ultimately

Brings back Prismo from the 'dead'

Where did they learn this?

Prismo refers to it as his plan B. Is it this song that is responsible for the events of the episode or is it something about

Prismo's pickles?

Which is implied by the lyrics:

Fallen Ally,
Fallen Ally,
Home now in the sky.

Fallen Ally,
Fallen Ally,
A single tear we cry.

A truer friend there was none,
and Prismo was his name.
An artisan of pickling,
And now just one remains.
And now just one—

Prismo, we'll love you forever in our dreams.
Prismo, we'll love you forever in our dreams.
Prismo, we'll love you forever in our dreams.

Pickle rama,
Pickle rama,
Pickle rama,
PICKLE RAMA!

Answer 1

Based on the dialogue with Prismo later in the episode the most likely explanation is that the song and ritual are simply Finn and Jake's own little performance and Prismo's plan was actually activated by the consumption of the last pickle that he had given Jake. Since Jake would only run out of pickles if Prismo was dead it makes sense that he was transported away after eating the last one, as Prismo had planned as a back up.