How does a leadership challenge to a Conservative prime minister work?

Prime Minister Theresa May's position as leader of the Conservative Party - and consequently her premiership - is under threat, with media reporting that some MPs who are unhappy with the draft Brexit deal she has reached with the European Union are taking steps to call a confidence vote.- WHAT NEEDS TO HAPPEN FOR THERE TO BE A LEADERSHIP CONTEST?A challenge is triggered if 15 percent of the Conservative MPs write letters demanding a confidence vote to the chairman of the party's "1922 Committee