Start out slow and easy. The first attempts at freestyle rapping do not even need to rhyme. The main focus should be to let your ideas flow out in a timely manner.
Keep trying. Most of the first freestyle attempts will not sound very professional. Try not to get discouraged when you stutter or cannot think of any rhymes. Keep flowing.
Try to find your next rhyme ahead of time. When you have thought of a line and know which ending rhyme you will use, immediately start thinking about the ending rhyme in the next line.
Write down rhymes in your spare time. This practice will embed rhymes in your memory and they will be easier to think of in the future. This is an invaluable practice when learning to freestyle.
Use your surroundings to find different flows. When starting out, observing things around you can make it easier to find rhymes.
Find groups of two or more people to freestyle rap with. When freestyle rapping with others, you have time to think of your next rhymes while they are rapping.
Practice freestyle rapping during any free time. The best way to get better at something is to practice as much as possible.