Firstly why do you want to do it? If it is as an experiment, it is probably not a good idea.
If you have something you really need to fast and pray about, here are a few tips. (I have done 7 days on water alone).
You will probably get a headache, and sometimes a bad one. if you press on, it will go.
You will get very hungry on the second day or so, but drink water in small controlled amounts.
Don't do this if you are doing physical activity.
Really, don't do this if you have any sort of medical condition.
If you do start eating after a day or so, take small amounts of non fatty food like crackers or something similar to begin with. Not too much salt either.
If you start feeling dizzy or lightheaded, were it is affecting you negatively, stop straight away.
Tell someone you can trust what you are doing (this is purely a safety thing)
Best to try it with a few people for support the first time.
Stopping a fast for health reasons is not a sign of a lack of faith, it is common sense.
Pray and meditate continuously, that is why you are fasting, to clear your mind and worship God. don't strive for answers. If God wants you to know something He will tell you.