Start rhyming some easy phrases. Just put some simple words together rhythmically and let the flow create itself. Don't be too critical of the content at this stage.
Keep flowing and rhyming. See how long you can go before you run out of words. Don't be afraid to make mistakes; A lot of your rhymes may be simple or sound stupid at first. The key is to just let it go and get used to flowing and using words in creative ways.
Learn how to use fillers. A filler is a little phrase you can insert occasionally if you need more time to think of a cool line. Come up with a few filler phrases that sound cool that you can insert at moments where you need more time or to bail you out of tough spots. As you improve your skills, you'll rely less on your fillers.
Create rhymes in your mind ahead of time as you flow. Try to think ahead to the next few phrases to find words that relate, rhyme and connect with the content and flow of the phrase or verse your on.
Write out your raps and flows. Writing rap down will help you to embed flows and fillers in your head and learn how to connect words together better. Writing will improve your freestyle flows and also expand your rap vocabulary of words that rhyme well.
Pick good content. The best rap flows include references to things around the rapper, metaphors, and references to current world events.
Pass the mic. Rap with friends and pass the mic around between verses to flow off each other's ideas. Listen while your friends are flowing and think ahead to what you want to rap about when the mic comes back around to you.
Practice! You need to practice freestyling the same as you need to practice doing anything to get good at it. So practice, practice, practice until your skills improve and you can flow easily at a moments notice. Try to make it look easy; As your skills improve your flows will be much more seamless and smooth.