How to write my common app essays, extracurricular and other recommended essays with authenticity

Well your Common App essay is supposed to be a very personal essay which let admission officers know about who you are as a person. You should be picking an event or discussing something that occurred in your life and describing how that event impacted you as a person and made you who you are. Essays like that should be pretty authentic by default because you're describing something personal to you and the emotions and impacts that event/experience/whatever you're writing about had on you. Admission officers have read a ton of admission essays and, while I haven't, I would bet they can very easily tell when an essay is authentic or not. It's very difficult to replicate the thoughts, feelings, and emotions of some things without having experienced them yourself. As long as you are writing about something you experienced yourself I don't think it will be difficult to make it seem authentic. If you're worried about how the essay is I'd recommend submitting it to our Peer Essay Review and see what other people think of it.

In terms of other recommended essays I'm assuming you're talking about supplemental essays that schools want you to write? I think many of the points I made still apply here. Take for example a "Why this school" or "Why this major" essay. These essays can be difficult to write but at the end of the day the answer is going to come from you and include authentic reasons why you want to go to a specific school or study a specific major. You just need to dig deep, get a bit introspective, and figure out the reasons why. Maybe you want to go to a specific school because of certain opportunities they have, certain professors, location, etc. You might want to study a specific major because you want to help carry on a family legacy/business, or because something happened and you want to make sure it doesn't happen to other people, or any other number of reasons. It might help when you're browsing through schools to make a list and write down the reasons you are drawn to a school. It can make writing these essays easier when the time comes.