Here is a quick rundown of several kinds of fasts. Choose carefully and prayerfully, and remember that this is between you and God. If a friend is fasting with you, each of you may choose a different type or length of fast There is no wrong way to do it as long as you are doing it under God's leadership and for the right reason: that is, to deepen your relationship with him.
A TOTAL FAST means no food or water. Personally I would not try this unless a doctor aimed a loaded syringe at me and told me I had to or die. The thing about a total fast is that it can be very dangerous, and fasting for spiritual purposes should NOT cause you harm. I won't dwell on this one, just wanted to mention it.
On a NORMAL FAST you drink lots and lots of water and go without food for a specified number of days.
On a PARTIAL FAST you may refrain from eating certain foods or you may fast one meal a day. I have a friend that has been fasting one meal a day for several years as she prays for the salvation of her children.
When I fasted last January I decided that I would do a normal fast for the first three days and then a partial fast after that. I drank water for the first three days, then after that ate fruits and vegetables, a little cheese, nuts. That's about it. I also decided that each Monday would be a 'water only' day. I continued that practice for about 5 months after the 21 day fast ended.
There was no special significance to the 3 days of water at the beginning of the fast. It was just something that seemed to be on my heart so I did it. Continuing the water only Mondays was just a great way to start each week.
I don't know if I could do a normal fast (water only) for the entire 21 days, and I'm not sure I'm going to try that this time. I'm still praying about that. If that seems like too much I may do it for the first week. The amazing this was that at the end of the third day of my fast I really felt like I could go longer without food.
Just like your salvation is a very personal thing between you and God, as is the way you live out your calling, this fast is personal. Don't get hung up on the details and become legalistic about it. Pray. Consider different options and what you feel able to do. It's fine to stretch yourself a little. It will lead you to rely on God's strength to succeed.
Also remember this: you cannot increase your worth to God by fasting. You can't make yourself more precious to him or more worthy of his love. Fasting is not about that. This is a way to meet him on a deeper level without the distraction that food often becomes.
I am praying with you about the type of fast to choose. Let me know what you think. How will you fast?
I am praying for you.