In my experience, many people don't know how to write essays. And for good reason! This is a fantastic opportunity to improve your application.
The essay should illuminate those traits of your character that standardized tests will not show in any way. Is 99papers legit? The goal is to show what kind of person you are and to emphasize your goals.
The essay needs a central story.
Each paragraph should have one key thesis statement.
The first and last paragraphs of the essay should overlap - there should be thoughts on the same topic.
You should characterize yourself favorably in the essay. When I wrote mine, I first told a fairly simple story about myself: that I was from the Donetsk region and wanted to win the Olympiad in history, but I only won 15th place. Then I found out that there is a teacher in Ukraine, which best prepares students for such Olympiads, but the way to her house from mine - a full two hours. That did not stop me, and I went to her to study in this small town. The effort paid off: I won the Olympiad and got into the university I wanted.
Your story, of course, will be very different. But it should have something that seriously grabs you.
It is also worth understanding how the admission committee works. First all materials are read by experts. They go through hundreds or even a good thousand stories. Then the admissions committee sits in a circle (I'm not kidding!) and begins "pitching" their best candidates. Each professional picks about 30 of the best stories and starts "promoting" them - briefly arguing why that person is worth hiring. To make that person you, write a story that the specialist can physically retell to others.
Hide the "hook" at the beginning of the story. You only have 2-3 sentences to keep your story reading carefully next, rather than just looking through with one eye. Grab the reader's attention, break patterns, and shock (but in moderation!) skillfully. Write original and cool in those first three sentences. It should hit you emotionally, but don't overdo it: "squeezing out" tears is a recipe for failure.