Even if I'm not that experienced mentor,I have checked some essays.I noticed so many mistakes one of them is writing all the experiences in one essay, you have only 650 words, sometimes it ends up with only your achievements, that's not the point! You have already mentioned your achievements in your application. Just imagine, admission officers have just checked your achievements then moving on your essay and your essay is also telling the same story, that's totally boring right!? You can see how I give feedback here (not professional feedback)
So read that book but pay attention if you don't do exercises in that book, you can't see any results! I will try to share it as a post)