Once I've got an idea in my head that I think is funny, and I can come up with a few "hooks" and rhymes, it's away we go! The basics of it, depending on length and complexity, sometimes I get in less than an hour. When my brain is dead from regular mental work, I can still do parodies. It's FUn!
The key is first having an idea I think is funny. If I can make myself laugh, I figure it will work for others.
If you are familiar with Rastafarian music (I'm not) then you might want to consider a song parody based on that music about Pitt called "Trustafarian Boi."
The closest I know to Rastafarian music is Calypso.