This is the best method of how to rap that i can tell you.

Step 1: Background/Clear Your Head/find a Beat

First you need to understand the difference between rapping and free styling.
Rapping is written and planned. Usually about what is on your mind.
Free styling, (which i will cover in another instructable),
Is not planned or written, it is thought of on the spot.
Usually to have fun and throw around insults
with no harm done. So with that, lets start!

Things you'll need:
A piece of blank paper
A pencil
A brain
A random thought

This is pretty much the 2nd hardest step but you have to do this first.
This also works for free styling.
All you need to do is try to focus on what you had on your mind.
Let the rhymes flow from that one thought.

You must remember that you have to find a good beat to go with.
The best method (if you're a beginner) would be to search for instrumentals
on the internet, or file sharing programs.
If you would like to make your own beats, I would suggest
Beatcraft.

Step 2: Let It Flow, Make It Rhyme

As the thoughts flow from your original, make sure you make them rhyme.
Put them into phrases. Make the last word of the first sentence rhyme with the last word of the second.
Make sure you don't stick with one rhyme sound.
(Example:Cat, Bat, Hat, Mat, Sat = NO!!!)
Try to change it up. Maybe end four sentences with the same rhyme sound.
Then change up your rhymes as u proceed through out your song.
Make the song as long as you want.
But the average time is usually 3-4 mins.
(If your a beginner, I doubt you'll go past that anyway)

Step 3: You're Almost Done(somewhat Optional)

If you like you can record your Rap along with the beat using the sound recorder that comes default with most PCs (I'm not sure about MACs)
And there you have it. You may edit to your liking by cutting and shortening it with windows movie maker or a different program.
GOOD LUCK!!!