It was authentically narrated by the Prophet , that the envious person should make ablution and allow the person affected by the evil eye to take a bath in the left-over water. Indeed He ordered the man who had envied Sahl to make ablution and give the leftover water for Sahl to make Ghusl (ritual ablution) with. [Maalik: Saheeh (authentic)]

This may be done by ordering the envious person to bring a container of water, submerge his hands in it, rinse out his mouth and spit the water back into the container, then wash his face, take water with his left hand and pour it over his right knee over the container, and take water a second time with his right hand and pour it over his left knee. After that the person should wash the edge of his loincloth and avoid putting the container on the ground. Then pour the water over the head of the affected person from behind his back. [Al-Bayhaqi]

A person should use this Ruqyah along with other invocations that were authentically narrated from the Prophet to treat the person afflicted by the evil-eye. 

Allaah Knows best.