1) Calculate how many calories you body needs to consume to maintain your existing weight. This is called you maintenance calories.(Search Google with "calorie needs" and you will find a few different calculators to help you with this)
2) Initially deduct 10%(to a maximum of 500Kcal)off the total maintenance calories. Drastically reducing your calories will result in muscle and water loss - not fat loss.
3)Divide you calories over 5 or 6 meals rather then 2 or 3. You should aim to eat every 3-4 hours.
4)Cut out all simple carbohydrates (except straight after exercise, eat whole grain, high fibre foods.)
5) Exercise at least 3 times a week, doing 30 minutes of cardio to a reasonable level of intensity. This will help to create the calorie deficit and encourage your body to lose fat.
6)Make simple replacements in your diet, Sweetener in place of sugar, diet drinks in place of full sugar drinks, use leaner cuts of meat etc. At first these changes may be difficult but you will get used to them and make vital calorie savings every day.
7) Drink plenty of water - at least 2 litres of water every day - if you are dehydrated you body will be less efficient and will make fat loss more difficult.
8) Try to eat low carb(complex only!), low fat and moderate protein. From my experience the South Beach Diet is excellent.
9)Keep a food diary to monitor how many calories you are consuming - it is very easy to over eat.
10) Supplementing you diet can make fat loss easier. A diet high in protein can help preserve muscle (more muscle means an increase in metabolism). There are also some fat loss supplements available which help boost your metabolism, this is useful as your metabolism may slow during extended periods of dieting.
This article is written from my own experiences. I have lost 20lbs of fat and gained 20lbs of muscle. My body fat is 11%. Please consult a doctor for proper medical advice.