Informal mentoring is a natural process where information and new skills are passed on. It usually exists between a supervisor and a subordinate. It happens naturally when we find a new way to do something or show somebody a new method. Quite often we don't even realize that we are mentoring someone because it seems so natural. Formal mentoring is a more structured environment where knowledge and skills are formally taught.
Persons have a more defined role of teacher and student.
Take for example a Coast Guard person who is completing the qualification standard for boat crew. From the beginning a formal mentor is assigned in order to carefully teach the required tasks. Look for someone who has the expertise and experience that you would like to emulate. Initially, you may want to approach them with a specific professional question. If that goes well, ask them if they have the time and would be willing to be your mentor and share the benefit of their experiences with you.