The idea is to practice using slow talk during your therapy time (while you are reading out loud), and then after doing this for a week or two, gradually bring it out with someone easy to talk to. Once you get comfortable using slow talk with this easy person, then move it up to another easier person, and continue on in this step-by-step fashion. Eventually you will be able to use slow talk with an authority figure or someone that causes you higher amounts of anxiety.
You do also likely have plenty to talk about. This is likely an ANT that you are telling yourself you don't have much to say. If you haven't gotten to this point in the series, you'll learn how to change this type of thinking around so you can see that in fact you do have a lot to talk about.
If you're unable to talk at all in current conversations then just keep practicing on slow talk in your 30 minute practice time and once you get the point of the series where you can turn your negative thoughts around, you can then start to bring slow talk out to the real world.