Just Write

Select a vivid memory and write about it. Write everything that comes to your mind. Don't worry about grammar or even the coordination of thoughts. Just be concerned with getting this memory on paper. Then put the writing away for a minimum of a week. In the meantime, you can write another memory on another day. Continue doing this. When at least a week since writing the memory has passed, (longer is good) bring it out and read what you wrote. It's likely that new thoughts about the event will surface. You can also work on polishing the grammar and sentence structure a bit to give it a story feel, rather than random thoughts about the subject.