Here are some relatively simple guidelines for a good parody structure. Of course, rules are meant to be broken, so don't be bound to all of these, but also make sure you have good reason if you want to try other things.

1. Pick a funny, strange, or current events topic. Or all three if you are lucky enough to find something like that.

 

Example 1: Funny/strange:

Something that just has no reverence to everyday life, just a silly song about something simple: Chinese food, bowling, mullets... The key to these types of parodies is simple but also challenging: Keep lines of the parody as similar as possible to the original lyrics. This way, people hear that the song is so similar, yet these new words are just different enough to give it a completely different meaning. Just keeping the same amount of syllables isn't quite good enough for top-quality songs. For example, 'Proud Mary' by CCR is famous for the line "Rollin' on the river". A good (though not necessarily the best) substitute for this would be the obvious "Bowlin' on the river"; a line like "Goin' to the circus", while having the correct amount of syllables, just doesn't have the right similarity to the original to get people laughing.

 
Example 2: Current Events:

This one is best to pick the topic first, and a common topic these days might be "Osama Bin Laden". Following the guidelines from above, we need to find a song that could fit one or more of those words into the title or lyrical hook. "Rosanna" could easily become "Osama", "I've been crawling in the dark" could be "There's bin laden in the park" (bonus points for having 'bin' and 'been' so similar). One good way to fit political events into a song is just listen to a song until you can think of a topic that works.

2. Once you have the song and topic chosen, listen to the song a few times, thinking of any possible lines that can go with the title, that also fit in well with the song. This doesn't just mean the main lines, just think of some good ones that you plan to use later on.

3. Next you start writing. One way I like to write is to listen to the song one line at a time and write the new words, or open a text document with the original lyrics and replace them as you go along. Listen to the song when you think you are finished to make sure you didn't miss any parts. Usually you may find one or two lines that don't fit especially well, and just don't run smoothly. Keep trying different words and combinations until you think what you have is just right.

4. Finishing touches. Listen to the song while reading the parody and go over it one last time, making minor changes and adjusting typos or spelling mistakes.

Tips:

a) Make sure everything rhymes, with the original, and if not the original then rhyming in parts where the original did so.

b) Try not to write a parody that people would find offensive, for example an insult to someone who is likely to read it. This includes racist parodies, etc. If you think your topic might be offensive (like making fun of vegetarians for example)

c) Don't do something completely pointless or stupid, unless you have a lot of funny ideas for it. But a parody about dogs or something, could be funny if there are a lot of lines you could parody in the song, rather than just the title (listen to "Stinkin' Up My Backyard" by Bob Rivers and Twisted Tunes for a good example). Be careful, these parodies can also be pretty boring.

d) Using lines from the original song can be very effective, and also very bad. People who know the original very well will know what lines are the same, and the same words applied to a completely different topic can be very funny. But keeping the same words in the same context is a waste of a line that could have been funnier.

e) Try to make Every line as funny as possible. Don't use a bunch of boring lines just to lead up to a couple funny ones.

f) Make sure all your syllables are correct. Keep the right amount of syllables in every line (with the exception of leading up to a line). More importantly, keep the same syllabic structure if possible, meaning replace two syllable words with two syllable words, etc. This is not necessary, but makes for just that much better of a parody. Finally, we want the rhythm to be pretty much exactly the same. In other words, we want accented syllables to be replaced with accented syllables, and unaccented with unaccented. Example: "I Heard it Through the Grapevine" was done as "I Heard they Have a Grape kind" in one of my parodies. These accents are the same. "I heard George just turned nineteen" doesn't work properly, because the reader expects to read it as "I Heard George Just turned Nineteen", which takes the attention off of the important words.

Finally, good luck, and I hope that you're learned something from reading this.