The rules of playing free slots are the same as for the normal slots one plays at the no deposit online casinos. The only difference being that one does not have to deposit any money while playing the free slots. Obviously, one should also not expect any monetary prizes too, since they are playing the free version.
If you are satisfied with the free slots, you might want to take a look at the paid section where you can win money using the experience you have gained. However, there are some people who are not confident about their playing skills. If you still lack in experience, you can keep on honing your playing skills at the free slots section in the online casinos.