The major difference between a memoir, and an autobiography is that the former is narrative and not descriptive. A memoir is not only about narrating a particular incident, but also the author's attitude and feelings towards it, while writing.
For instance, a memoir can be written about one's relationship with his mother, recounting several important incidents that added meaning to their relationship.
A memoir can be a particular incident, or a series of events that has fascinated or changed the life of the author. However, it must be written like a novel or a fiction. It should have all the literary elements like plot, setting, scene, characters, dialogs, etc., which would make it interesting for the readers.
Adding details like smell, colors, sounds, etc. is important to create an imaginary atmosphere of the incident, for the audience.
It is very difficult to write about one's life truthfully. You may have to reveal your inner feelings, negativeness, and weaknesses, which is not an easy task. It is very important to be honest and truthful to yourself.
When you decide to pen down an incident in your life, the first thing you should do is brainstorm and create an outline. You may not remember every detail of the incident like the weather, color of the walls, or the exact dialogs. But you could take help of others who were present when the incident took place, like a family member or a friend.
However, remember that every person has a different interpretation of the situation, and may also remember it differently. It is very essential that you do not get influenced by their thoughts and attitudes, about the incident. The memoir should only reflect your perspective and attitude, towards that incident.
A memoir can be about a negative or a positive incident in the life of an individual. You can write it in first, second or third person narrative. It is a recall of an important memory in the life of a person, and should be written in such a way that the reader understands the writer's inner feelings and thoughts.
Writing a memoir is said to work like a therapy, if it's about a crisis in the person's life. Because, while writing, he is reliving that particular stressful incident or event, which may help him relieve his tension and pain, inflicted by that situation.