When you first start trying to rhyme in a freestyle form you can't be afraid of sounding dumb. Odds are you will let loose some rhymes that don't make sense, don't rhyme correctly or simply sound bad. That's okay as long as you keep working on it and get better. The key is to keep it simple. Don't try to jump right out with complex rhyme schemes and big words that are difficult to rhyme. Don't be afraid of sounding like an amateur, because everyone does at first.
You must ignore the urge to stop when you mess up. If you stop your flow because you goofed up a rhyme, there's a good chance you're going to get stuck. This is the worst scenario. It's better to keep it going and work through the mistakes than to bail out in the middle of the line.
Practice makes perfect is something you have most certainly heard about many things throughout your life. When it comes to freestyle rap, the rule is the same. The only way to get better is to keep practicing. Get in front of a mirror and just go for it.
A key to learning the skill is to always think ahead. When practicing freestyle rap, always think of the last word in the line you are saying and go ahead and start thinking about the word you'll use at the end of the next line. This will take some time to develop, but before you know it you'll be able to multi-task in your head and rap while you think of the upcoming rhyme.
As a general rule, freestyle rap artists don't write their rhymes out ahead of time. The whole point of freestyle is to show you are able to come up with great words on the spot without preparation. But don't fool yourself and think that the best freestyle rappers never sit and write rap lyrics. They have to in order to get good. The more you write rap lyrics, the fresher the rhyming words will be on your mind. You may not use the exact lines you wrote down in an earlier session, but you may use the same rhyme schemes or rhyming words in a different context. You may even need to pull a line from a song you write as filler in a rap battle.
You should rehearse your written rhymes. Use the pre-written lines to rehearse your act to get the sound and the movements you want fine-tuned. You can more easily work on these details when you aren't too busy trying to come up with rhymes in your head. Eventually they'll all come naturally at the same time.
The content of your rhymes is as important as the rhyme scheme and the performance. You should be able to prove you are freestyling and not just repeating written verses. To do this you can look around the room and try to work people and objects in the room into your lines. This will impress the audience and show that you are able to grab details they can see and instantly work them into the act.
When you are practicing and writing rhymes, always think about current events in the news or in pop culture. This will keep your rhymes fresh and current and keep you from talking about mundane topics.
Find a friend who is interested in freestyling. You can stage battles against one another at home and get used to the method before taking it public. This will help you both build confidence before stepping up to another rapper in a freestyle battle. Practicing in a group is also a good idea.
One of the key elements to winning a rap battle is winning over the crowd, and often this is best done by "dissing" or disrespecting the other rapper. It's not always done in a mean way. It can be poking fun at the other guy harmlessly, but always use your practice sessions to diss your friends to try the style out and hone your act.