https://onepeterfive.com/fast/

For 40 days, from Ash Wednesday (2/22) to Holy Saturday (4/8)

• no meat, no fish, no shellfish, no dairy, no egg (basically vegan)

• fasting is defined as only 1 vegan meal a day Mon-Sat

• Sundays are the exception and allowing for 3 vegan meals

If that is too difficult, then modify it to suit your personal needs (eg. give up meat but let yourself eat dairy, only fasting on Mon-Wed, etc.). The point is self-sacrifice and penance, not making yourself completely miserable, so as to cast out the demons, beg God for mercy, and placate His wrath.

https://www.biblegateway.com/passage/?search=mark+9&version=KJV

When Jesus saw that the people came running together, he rebuked the foul spirit, saying unto him, Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him. And the spirit cried, and rent him sore, and came out of him: and he was as one dead; insomuch that many said, He is dead. But Jesus took him by the hand, and lifted him up; and he arose. And when he was come into the house, his disciples asked him privately, Why could not we cast him out? And he said unto them, This kind can come forth by nothing, but by prayer and fasting.

https://www.biblegateway.com/passage/?search=Jonah%203&version=NIV

Then the word of the Lord came to Jonah a second time: “Go to the great city of Nineveh and proclaim to it the message I give you.” Jonah obeyed the word of the Lord and went to Nineveh. Now Nineveh was a very large city; it took three days to go through it. Jonah began by going a day’s journey into the city, proclaiming, “Forty more days and Nineveh will be overthrown.” The Ninevites believed God. A fast was proclaimed, and all of them, from the greatest to the least, put on sackcloth. When Jonah’s warning reached the king of Nineveh, he rose from his throne, took off his royal robes, covered himself with sackcloth and sat down in the dust. This is the proclamation he issued in Nineveh: “By the decree of the king and his nobles: Do not let people or animals, herds or flocks, taste anything; do not let them eat or drink. But let people and animals be covered with sackcloth. Let everyone call urgently on God. Let them give up their evil ways and their violence. Who knows? God may yet relent and with compassion turn from his fierce anger so that we will not perish.” When God saw what they did and how they turned from their evil ways, he relented and did not bring on them the destruction he had threatened.