Choose your song and topic. You can write a satirical song about politics, current issues or pop culture. You'll want your tune to be recognizable. Jokes work best if the audience can refer to the original lyrics, so choose a well known song. Story-style songs often are easier to work with: The Boxer by Simon and Garfunkel, Don't Stop Believin' by Journey, or Complicated by Avril Lavigne, for example. If you're focusing on a specific topic, it can be a song with more repetition.
Get all the lyrics to the original song. Go over the lyrics a few times to get a feel for possible alterations. You'll want to preserve a lot of the song so that it is still recognizable, especially the line length. Don't try squeezing too much in; instead, figure out how to get your point across with less. If possible, keep the original rhyming structure as well. In other words, if a verse has two lines rhyming with "fall," try to make your lyrics end in crawl, stall or another rhyme to match. Don't limit yourself to one syllable rhymes. Use an online rhyming dictionary to find the perfect match.
Gather material. A good parody is successful because it works on several levels. If your new song is about the sexual misadventures of a celebrity, don't limit yourself to restating the obvious events over and over. Small details and relatable comparisons keep satire fresh. If you are playing off a pop culture phenomenon like Harry Potter or Star Trek, fans will appreciate obscure references. These may also open the doors for new rhyming combinations. In political satire, give enough information about the politician or situation in question so that even those without a savvy political background can get in on the joke.
Create the chorus. Once you have one section you're comfortable with, the rest of the lyrics will come to you more quickly. Listen to the original a few times, and fiddle with your words until they are clever and fit smoothly into the music. Then, build the verses and bridge from there. Decide if you want to tell a story with your song, like Weird Al's "The Sage Begins," or just create a series of one-liners. Be creative, and if you get bogged down walk away from the song for a while. Humming it in your head as you do other things may give you more inspiration than focused concentration.