There are three parts to operating outside a circle of competence successfully:
Learn at least the basics of the new circle you'll operate, and keep in mind that there are many many things that you won't know and don't know that you don't know. Basic information that is easy to obtain often gives you unwarranted confidence.
Talk to someone whose circle of competence is strong. Do initial research to define some thoughtful questions to ask and what information you need. Keep in mind to consider their circles and situations because it might influence the information they provide you.
Use other mental models to augment your limited understanding of the field you're entering. They'll help you identify the foundational concepts that would be most useful.