In fact, when I come up with an idea for a parody I try to resist the urge to Google the idea to see if someone has done it already because the answer is almost always, "Yes, of course they have, they've thought of it!"
I write and write and write, and then I edit it down to the parts that I think are amusing, or that help the storyline, or I'll write a notebook full of ideas of anecdotes or story points, and then I'll try and arrange them in a way that they would tell a semi-cohesive story.
I cut my teeth playing rock songs on the accordion when I was a teenager and my friends always thought that was extremely amusing. I think that was the genesis of my polka medleys, because every rock song I played on the accordion just sounded like a polka and my friends thought it was funny. So that was a joke that I continue up to this very day.
When I was a kid, I thought I was going to be an architect, because when I was 12 years old I had a guidance counselor that convinced me that that was the best career choice for me.