Please advise me regarding the most effective way of using the oil of the black seed in guarding oneself from the devil and thwarting magic spells?
May Allaah reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

We ask Allaah The Almighty, the Lord of the Mighty Throne, to grant you good health and cure you, and guard you from the evil and whispers of the devil.

Black seed can be used in many different ways:

First, you can read the Quran and Ruqyah (curing by means of reading the Quran and Prophetic supplications) on the oil (of the black seed) and then smear, eat or drink it.

Second, roasting the seeds then using them soaked or hot, eating them alone or with something else.

Third, mixing the oil of the black seed with honey, Zamzam water and reading the Ruqyah on it. In this way, you will get the blessings of all such things, which is also the best way of using black seed.

Allaah Knows best.