Start speaking in rhymes as often as possible. Instead of saying, "I'm going to the store," you may say, "My shoes are gonna hit the floor and take me to my destination, which is the store." Keep practicing until rhyming comes naturally, and then practice more.
Listen to your favorite rap artists and take note of the rhythm they use. Speak in a similar rhythm when practicing your rhymes. Add some lines that don't rhyme, but still speak in rhythm.
Keep flowing through mistakes, which will undoubtedly happen. Incorporate the mistake into the next line. For example, if you say a word that doesn't make sense, address it in your next line by saying something like, "Yeah, I'm so fly, I make up words on the fly."
Think ahead to the last word you plan to use in a line and start thinking about rhyming words. You'll need to think fast, as you won't have time to stop rapping.
Include metaphors in your rhymes. For example, don't just say "I'm sharp," say "I'm sharp like a Samurai sword."
Pay attention to current events and think of ways to include them in your rapping. You may compare a pop star gaining weight to you gaining fame and popularity. Keep these things in mind the next time you practice free-styling.
Practice with your friends after you feel comfortable rapping alone. Listen to their rhymes and think about how you can improve your own. Continue practicing alone and with friends until you are a skilled rapper.