The Sign of Jonah is the Fish of Jonah
Death and resurrection are part of our life in Christ. Dying to ourselves is the only way to live for God. Jonah was shown this truth through his ordeal in the fish. Jesus demonstrated it throughout his life, ministry and passion. We must die to ourselves so that we might live for God. Doing the will of the father requires it and it is a sign that points to Jesus Christ. It is a sign that signifies one is a disciple of the living God.
So, this is a fairly straight forward and easy sign. Jonah is swallowed by the Fish and remains in a death like state for three days. He rises on the third day with words of power to speak to the nations. Just like Jesus.