There is some good teaching on fasting in Richard Foster’s book, “Celebration of Discipline”.
It’s actually a very good book about practical things for Christians - fasting, prayer, giving, meditation, simplicity, solitude
It’s a good book to study with a small intimate group of people. Study one chapter a week together and then apply it.

In Matthew 6, Yeshua said “when” you fast, (not “if” you fast).
When you pray, when you give, when you fast.
He was talking to Jews. And he was assuming that they prayed, they gave to God and they fasted.

We would be shocked to meet a Christian that claimed they never prayed.
We would be shocked to meet a Christian that claimed they never gave to God.
But somehow we’re not shocked to meet Christians who don’t fast.

Another time Yeshua said that his disciples would fast after he was taken away from them.
He expected them to do it.
But somehow we’re not shocked to meet Christians who don’t fast.

But the Bible doesn’t teach us when or how often to fast. There isn’t any Bible commandment that people should fast.
The religious Jews used to fast a couple of times a week. But that looks like it was Pharisee law, added by man.
And anyway, we’re not under the law, we are under grace
Rules for Christian Living.

In the Bible, sometimes people fasted when they were mourning.
Sometimes they were really serious about some prayer and were asking God for something extraordinary.
They did this in the Old Testament, and in the New Testament, (Acts 13 for example).
So, presumably we should do that too when we have a big decision to make, or when we have a very important prayer.

(BTW: don’t fast if you’re diabetic or pregnant.)

But the Bible also doesn’t teach us how to fast. It was part of their culture in those days. Everyone already knew how to fast.

In its most basic form it’s pretty simple. Don’t eat anything.
In a normal fast, you can still drink water.
Some of my friends will still drink clear liquids when they fast (like tea, or even apple juice). But that is up to you.
Sometimes people fast completely with no food or drink. Don’t ever do this for more than a couple of days unless God specifically tells you to.
And by specifically, I mean using an angel or something like that, not just a “feeling” that God is telling you something.
It’s up to you how “complete” your fast is. I would only suggest that you decide what is and isn’t acceptable before you start fasting, and don’t let yourself change the rules half way through.

You can also fast from things other than food.
TV for example.

One of the benefits of fasting is that it reveals the things that are controlling you.
Paulus said that everything is permissible for Christians, but he said he wouldn’t do anything that would get mastery over him.
Many modern Western Christians are addicted to TV shows. They can’t go out on certain nights of the week.
If they do, they absolutely have to record the show so they can watch it later.
It is unthinkable to just leave the TV off and miss the show completely.
Try fasting from TV for a week. You’ll learn a lot.
If you’re under 20, try fasting from music for a week.

As mentioned, the main reason for fasting in the Bible was to show God you were really serious about some prayer.
It’s not that God owes you because you fasted. He doesn’t. And of course, God already knows whether you are serious.
But it’s a good way to show that you’re really serious about this prayer. (It’s a good way to find out too. If you’re not as serious as you think you are - you’ll stop fasting).

I usually fast if I have a big decision to make. And I really want to make sure that God has had a chance to influence it.
Or if there is something I am praying about which is really serious to me.
(Like when my son went to hospital when he was a baby).

Fasting does not make you more holy than someone who doesn’t fast.
Fasting does not make God happier with you.
Fasting doesn’t get you any “points”. It just shows God, and you, and the angels, that you are serious about your prayer.

Isaiah 58 is a good passage to read before you fast.
It’s about how God views our motives when we fast.
Fasting is not a way of manipulating God. It’s a way of getting your own focus back to where it should be.
If you’re fasting and God is not answering, check this passage. Maybe you’re fasting for the wrong reasons.

Fasting is sometimes hard. Especially if you’ve never done it. Your body is used to getting all the food it wants, and it’s not going to be happy when that doesn’t happen.
What you do not want to do is give up a fast just because your body is hungry.
So that will take some training (training your body).

Start by just fasting for one meal. Lunch.
Make breakfast as big as you want. But decide that you will not eat again until dinner time.
You are training your body, and your mind, that you won’t die if they don’t eat for a while.
Training yourself that you can go without food for a meal.

After skipping lunch once a week a couple of times, you’ll be ready for a bigger step. Skip breakfast and lunch. It’s OK, you won’t die.
(If you’re worried about that, or if you have any kind of illness, check with your doctor first to make sure).
This is basically a 24 hour fast. You eat dinner one night, and don’t eat again until dinner the next night.

After a few weeks of skipping breakfast and lunch once a week you’re ready for a full day fast.
Eat dinner the night before, but then don’t eat again until breakfast the day after.
If you worked your way up, this will be fairly easy and you’ll be very excited when you make it.
You’ll also enjoy breakfast like you haven’t for a long time.

After you have done this a few times you will be able to fast whenever you need to.
And you won’t have to keep doing it regularly like this anymore. (But that’s up to you).
You will know that your body is just wrong when its demanding food, and you’ll know that you can go without food for a couple of days.

If you drink coffee, just be aware that if you fast for more than a full day you’ll probably get a massive withdrawal headache around day 2-3. It will pass. But it will be painful.
You might like to stop the coffee for a couple of days before you start fasting.

I usually find that when I fast, if God is really in it, then I don’t feel hungry, even after a few days.
But your experience may be different.

If you decide to fast for more than 2 weeks, and after 2 weeks you start feeling hungry again, you should break your fast and eat.
Usually this is a sign that your body has started to digest itself. Not good. This time you can die.

And one final piece of practical advice. If you have any problems with constipation, you should make sure that your last meal before a fast of more than a day is fruit and vegetables.
Also, if you are breaking a fast of more than a couple of days, consider your body and give it easy to digest things like fruit juice to get it started again.

Fasting is not compulsory for Christians. But neither is giving to God, (you can be a Christian without giving anything, sort of).
Fasting will change your life.
So train your body to fast for one day a week for a few weeks and then you will be able to fast whenever you need to.
Always check yourself and your motivation. Remember Isaiah 58.
And when you fast, do it in secret, (at least as much as depends on you).