One really good way is to just start writing down everything you can think of that has to do with that personal experience: sights, sounds, memories, smells, and feelings. When you do this sort of brainstorming, you don't have to worry about grammar or even writing complete sentences. Just write a list of everything you can remember. Sometimes people make this using a web, with the main idea in the middle and lines going out to show the connecting ideas. Whichever way you write it, this brainstorm list gives you a start for your ideas.
After that, you will need to organize your information in order to write the essay. You can use the ideas in this article for that. You might also want to look at some of my other articles and my Personal Experience Essay example that is linked to this article. Another good article to look at is "How to Write a Great Thesis for your Essay."