Excerpt taken from Revival Starts Here by Dave Clayton.
Although the Bible often describes how people in the Bible would fast, it does not specifically prescribe how we as Jesus' followers should fast. Jesus deals with our motives, but he does not address the mechanics of fasting.
Below is not a prescription of how you should fast; it's simply a description of several things that could help you step into a season of prayer and fasting more faithfully and effectively.
Generally speaking, there are four common ways Christians choose to fast.
A major fast is when you abstain from food or drink for twenty-four consecutive hours or more. A major fast requires a certain level of physical health, and it's recommended that you check with your doctor if you're doing a major fast that lasts more than a few days in a row. Some examples in scripture of a major fast can be found i Matthew 4:2, Acts 9:9, and Esther 4:15,16, just to name a few.
In a minor fast, you choose to abstain from food for a certain period of time (i.e. 6 a.m to 3 p.m. or sunup to sundown). This fast is sometimes called a "Jewish fast," and it can last for days at a time, eating only a set time of the day. In a minor fast, most people will drink liquids during the day and have one small meal after sundown each evening.
A partial fast is when you give up only certain foods during the course of your fast. This fast is seen most clearly in Daniel 1, where Daniel abstains from certain foods out of devotion to God for a period of time. During a partial fast, some people choose to give up certain foods they enjoy in an effort to turn their hearts more deeply toward God.
Technically speaking, biblical fasting only refers to abstaining from food or drink (or both) for a spiritual purpose. However, some choose to fast from things other than food. Some people give up television, social media, or other activities for a soul fast to focus on prayer.
Begin by asking God to help you understand why you are fasting.
Chose what type of fast, what you will fast from, how long you will fast, and with whom you will fast.
Give your mind, body, heart, and community adequate time to prepare for the fast.
Make a plan for what you will do with the time normally given to having a meal or whatever you are fasting from.
Start your fast, and try to stick with it. If you break your fast along the way, simply start again.
Share your plan with your community or at least one other person so that you can encourage each other throughout the fast.