Before you create an intervention when an autistic student talks to themself, you need to ask yourself why they are doing it. If you can learn about the specific triggers that cause the student to retreat into their own world and talk to themselves, you may be able to prevent it next time and you may be able to teach the student some effective coping skills so that they can learn how to handle such situations in a more appropriate manner. Some triggers may include loud, high pitched or repetitive noises, a change in schedule, the presence of something that frightened them or adverse peer interactions just to name a few.
To stop the self talk, you might first consider removing the child from all stimulation. Use tried and true calming techniques that have been developed to fit the need of the child. One great technique is a time-out. There should be a time-out area created just for the purpose of calming the student down. This might also be known as a sensory area where the student can use sensory methods that help to calm him or her down. The idea is not to punish the child, but to give them a chance to calm down in a removed situation.
As part of the intervention, the autistic child may need help understanding what happened or what caused them to do this activity. That's why it's important for you as the teacher to record such events and use them later in discussions and story boards so that you and the child can review what happened and why. Use the story board to discuss the specific events and alternatives to self talk. Develop signals or other methods so that the student can express to you that they are being over-stimulated or upset.
The main idea is to make the intervention a productive one. As time goes on and coping skills are developed, the use of those coping skills that replace self talk should be rewarded while self talk is discouraged. Keep in mind that these coping skills may take a while to develop and should be encouraged every step of the way so that eventually intervention when an autistic student talks to themself is no longer necessary.