Write the essay before you insert a thesis. Often, rather than working from an outline, writers of narrative essays discover what they have to say as they write. Writing the thesis last will prevent you from feeling you must write to conform to the thesis.
Read your essay with a mind to writing a thesis statement after you have revised it, and it feels complete. Ask yourself what your essay is getting at. Narrative essays always have a point, so chances are you can boil it down to a lesson learned. For example, maybe you wrote about a period of strife between you and your mother. You have always blamed her, but after having written the essay, you realize that the cause of the tension between you and your mother really was your fault and that you had been willful and disrespectful toward her. She had been more patient than you had realized.
Write various thesis statements until you find one that works. Maybe the first one will work or maybe the tenth one. Because many narrative essays try to sort out interpersonal or personal issues, think in terms of principles, personal morals, ethics and universal truths. For example, maybe you now realize that youth is blind. Or that youth is unwittingly selfish. Approaching it from a different narrative essay, perhaps one that concerns workplace events, you might develop a thesis concerning the value of minding your own business or the importance of intervening, rather than minding your own business. You can turn any insight into a thesis.
Place your thesis at or near the beginning of the essay. You might use it as a freestanding paragraph of its own, or use it as the final sentence at the conclusion of the first paragraph.