There are three dimensions to the use mindfulness with clients or patients: your own practice, teaching and being informed by your learning.
Practicing mindfulness yourself is essential. This practice may be formal - mindfulness of breathing, for example - informal (returning again and again to the awareness of mindfulness during the day) or a combination of both. It is only through practicing mindfulness yourself that you will experience the potential and, indeed, limits of this approach in therapy.
Teaching involves explaining principles of mindfulness or of the thinking behind it to the client. Teaching also involves giving the client simple mindfulness techniques to use. The brief mindfulness exercise at the top of this blog is an example of such a technique.
Finally, your use of mindfulness is informed by your learning and that learning is derived from practice and from theory. Reading blogs like this one is a help. So is knowing something about Buddhist psychology. So is reading my book - ok, and other people's books too!