In brief, a personal essay covers a lesson the author has gleaned from life experience. The writing style is usually conversational rather than formal. The whole point is to both give a detailed description of the experience chosen as the subject and for the author to reflect on the meaning they have gleaned from the experience after the fact.
Before writing, one needs to select an event or experience from the past to write about. It can be positive or negative in nature. Also keep in mind that the subject matter need not concern some great, life-altering event. A personal essay can be as small as recounting what one learned from taking care of a beloved pet dog during childhood or as large as facing mortality for the first time during the terminal illness of a loved one.
Once a subject is chosen, the author needs to describe it in detail. Using the childhood pet dog example, one would describe what the dog looked like, its personality, what games the writer played with the dog, what chores regarding the dog might not have been as enjoyable (such as letting it out to relieve itself in the middle of the night), etc. The more in-depth the author can be with the details, both physical and emotional, the more the reader will be immersed in the essay.
Finally, a good personal essay needs to include what the writer learned from this experience. Using the dog example again, perhaps this writer would discuss how caring for the dog taught them about how to be responsible, preparing them for adulthood. Or maybe the author had a bad relationship with their parents, and the dog was a source of love and comfort. It all depends upon what the writer has taken from this experience.