1) Relax! Fasting is an unknown for most people, a poorly-understood process for many who do know about it but something that you can quickly get to grips with through a bit of reading and asking questions on this forum. There are many other fasting support websites that have advice on hand such as http://www.fastingconnection.com/index.php.
As with this forum, there is a huge difference of opinion about different modalities and you'll find fasting experts advising putting various things into your body from different directions. My belief is that nature knows best and that is the Natural Hygiene proven principle. Avoid enemas, colonics, or the temptation to add anything to the water.
You can learn as you go but to get through the first 3-5 days, it helps to have the right mindset.
2) Preparation! You want to start immediately and have given yourself a days prep with just eating vegetables but you can do better. Taking another 4 days eating mostly fruit and a simple salad in the evening and on the last 2 days cutting your intake in half will reduce the shock of suddenly withdrawing food altogether. The prep itself will prompt the body to begin detox so you will get some noticeable changes during that period.
3) Drink only to thirst! Neither thirst nor hunger is felt in the stomach; that is only addictive eating urges, an attempt to fill the belly for comfort. Thirst is felt in the mouth and throat. Drink the water a swallow at a time and only continue if the sensation doesn't go away. During the fast you will probably get through 2 litres a day.
4) Set your goals! When you get tempted to break the fast due to foods on offer, negative vibes from family and friends, bodily discomforts, stop and meditate on the benefits you'll get from the process then smile and congratulate yourself for taking such a helpful journey.
5) Practice body awareness/Somatic Inquiry/or whatever technique works for you! Regularly or whenever you suffer discomforts physical or mental, sit quietly and focus on the feeling. Does it have a shape, colour, temperature? Is it connected to other parts of you? Can you follow it or sense where it's going. Don't judge what you find, just let it rest in your mind. If you then feel it is a cause for concern, address it to the forum or, if serious, to whatever professional seems correct.
6) Rest and sleep! Many people Water Fast while trying to maintain their normal activities. Fasting is the best method for giving the body a thorough rest which allows it to put everything into detoxifying and renewing the body in its neverending quest for ultimate health. The body has a limited supply of reserve energy, enough for 40 days easy. People have fasted for more than 60 days under expert supervision. If you remain active, that energy supply will be used up maintaining your activity and you will gain very little health benefit from the fast.