Follow all rules for normal dialogue. It is important that your normal, spoken dialogue is noted correctly so that telepathic dialogue is seen as different. This includes using quotation marks, using a new paragraph when you switch speakers and using speech verbs.
Use a clear distinction to mark a difference between telepathic dialogue and spoken dialogue. One way to do this is to put telepathic dialogue in italics, as opposed to quotation marks.
Introduce telepathic dialogue with a verb of thought, as opposed to a verb of speech. Instead of saying "he says," use a verb like "thinks" or "projects."