How can we stop overthinking? Many people suffer because of an endless mind-thoughts. What is the simplest way to stop overthinking and make the mind still?
As a person with obsessive thought disorder I might shed some light on this issue.

The very fact that you are overthinking is that you are clinging to resolving an issue you believe is of highest importance at the moment, as Andrei Volkov already wrote. 

What you might do is to be mindful of obsessiveness as it comes. Label it "Obsessiveness." and continue with your mindful gate keeping. Don't let scattered thoughts (vicàra) roam freely unnoticed. Try to let only wholesome and directed thoughts (vitakka) free. It is the case of a habit. 

It is mostly the issue that such thoughts of overthinking take on subjects that aren't important or even difficult to solve with calm and clear mind, so along with mindfulness of thoughts please add concentrating on breathing. Do not try to tackle such thoughts in the state of confusion, as you will be like a man standing on the deck of the ship in the middle of the sea storm - you don't want to be out in such conditions, it is far better to keep yourself safe inside the ship and wait for better conditions.

No matter how tempting it is to continue inner chatter or how critical it might seem - let go of the need. As what they say: Don't try to talk with the madman - it is not going to work.

Bonus: Ajahn Sumedho - Let Go, Let Go, Let Go