What is the right way to fast?
The Bible doesn’t say much about how to fast. It simply seems to assume that anyone who is going to fast knows how. The words translated “fast” or “fasting” have the meaning of abstaining from food, or from food and drink. Generally speaking, those who fasted in the Bible did not eat or drink during the time of the fast.
“Then said his servants unto him, What thing is this that thou hast done? thou didst fast and weep for the child, while it was alive; but when the child was dead, thou didst rise and eat bread.” (2 Samuel 12:21)
“And while the day was coming on, Paul besought them all to take meat, saying, This day is the fourteenth day that ye have tarried and continued fasting, having taken nothing.” (Acts 27:33)
“Then Jesus called his disciples unto him, and said, I have compassion on the multitude, because they continue with me now three days, and have nothing to eat: and I will not send them away fasting, lest they faint in the way.” (Matthew 15:32)
Frequently, in the Old Testament, fasting is accompanied by wearing uncomfortable clothes (sackcloth), but this may just be an additional sign of mourning. It is often also accompanied by prayer.
There is only one fast required of anybody in the Bible. The Jews were required to fast on Yom Kippur, the Day of Atonement. That requirement was never made for non-Jews.
In the New Testament, fasting is almost always accompanied by prayer. It is seldom mentioned in connection with mourning, but sometimes mentioned in relation to preparation for making a decision. (Acts 13:2; 14:23)
The current practice among Jews is to eat or drink nothing for the 25 hours of Yom Kippur. Muslims, when fasting for Ramadan, merely refrain from food or drink during daylight hours, but break the fast at night. Roman Catholics, I am told, fast by refraining from food but not from drink. Even among Jews, those who are weak or very young are not required to fast, and especially not if it puts them in a life threatening situation.