Write a list of the most influential experiences you've had with this person. This will help you to gather your thoughts and decide on a thesis statement, which is a statement that tells the reader what your essay is about and what you hope to prove. After reviewing the most important events, does a theme emerge? For example, was your grandfather a strict disciplinarian? Did you resent it growing up but looking back you realized it had a strong impact on your life?
Write a list of descriptive elements about this person. Try to use all of the five senses. Also think about how you want to portray this person through these descriptive elements. Your grandfather may have dressed like a lumberjack and worked 12-hour days, but he may also have fostered homeless animals. Effectively showing contrast will give your essay complexity and richness.
Write an outline of your essay that includes your thesis statement and a numbered list detailing the order of the experiences and descriptive elements that you will weave into the essay.
Write a first draft of your essay, following your outline. List your thesis statement in the first paragraph, then draw upon those experiences and descriptive elements to create vivid scenarios in which the reader will feel that they have come to know the person and understand precisely why this person was influential to you.
Revise your draft multiple times. Do your descriptions show, not just tell, the story of this person's influence on your life? Could the reader visualize this person if they had to? Do the events of your essay flow logically and support your thesis statement?
Give the essay to friends to read. Have them give you overall impressions and what they felt was the main point of the essay. Ask them what aspects of the essay were the most memorable or compelling and also which parts confused them or need additional work.
Revise your draft again, if necessary, based on your friends' observations.
Edit the final draft for grammar, punctuation, and style.