Albert Barnes Commentary Deuteronomy 13:5

"And that prophet, or that dreamer of dreams, shall be put to death, because he hath spoken rebellion against Jehovah your God, who brought you out of the land of Egypt, and redeemed thee out of the house of bondage, to draw thee aside out of the way which Jehovah thy God commanded thee to walk in. So shalt thou put away the evil from the midst of thee." — Deuteronomy 13:5 (ASV)

The context and parallel passages (Leviticus 20:2) indicate that there was to be a regular judicial procedure, and that the manner of the execution was to be by stoning. In this the community was to take its part in order to show its horror at the crime, and to clear itself of complicity in it.