Also, keep in mind why colleges are asking for an essay: they want to get to know you better. Nearly all selective colleges and universities (as well as many that aren't overly selective) have holistic admissions, and they consider many factors in addition to numerical measures such as grades and standardized test scores. Your essay is an important tool for presenting something important to you that may not come across elsewhere in your application. Make sure your essay presents you as the type of person a college will want to invite to join their community.
There are a number of ways to approach freewriting, and all of them are meant to keep you limber, loose, and free. You want to sound authentic in your essay—which means stiffening up under pressure, as many students do, trying to sound formal, more stereotypically “adult” or “learned.” The more you can sound like you while freewriting, the stronger position you’ll be in when it comes to drafting the personal statement.
Remember that stories don’t begin with a repetition of the prompt (e.g., please don’t start with, “One time when I questioned or challenged a belief or idea was …”) or a definition from a dictionary (e.g., avoid saying, “Merriam–Webster defines ‘success’ as …”); instead, you should begin with something descriptive, such as setting the scene or jumping right into the middle of the action. Then, go on to illustrate how the event took place, devoting details only to significant moments. (Life hack: Keep in mind that this is also a story and not a novel, so don’t go all Charles Dickens on this.)
You may not be thrilled at the prospect of spending the summer before your senior year on college applications. But getting going in June after your junior year and committing to a few exercises over the summer will be like spring training for summer athletes. By the time you get to August, when you’re drafting your Common App Essay and your secondary essays in full, you will be warmed up, and much of the hardest work—that is, the reflection, figuring out what you want to say—will be done for you.
There’s no reason to rush your essay. You won’t score extra point with the admissions department for finishing your essay in an hour. Unless you’ve helped write the State of the Union, your admissions essay will likely be the most influential essay you’ve written so far, at least as it relates to your own life. Give yourself at least a week to compose your essay.