One should think logical and practically before jumping into unknown terrains. It's always better to know your body and it's nature as each individual is born with a unique identity (due to the role played by genetics).
The metabolic rate differs from person to person and so there cannot be a one plan that fits all regarding weight loss or weight gain.
Some important points to be noted.
Always check with your physician and see that you are fit to reduce or gain weight.
Infact it very important to confirm that you are overweight or underweight before you start anything!
See that you do not suffer from thyroid problems, diabetes or hypertension or any other illnesses.
Ensure that any change in lifestyle (including dieting and exercising) is comfortable for you and does not cause pain or discomfort.
There are no short cuts for success. It can be dangerous to use supplements or steroids, or even thinking of starvation or avoiding a particular nutrient in order to achieve fast results.
Use common-sense in taking important decisions. If some body promises miraculous changes without a scientific basis and that too in a short period if time, be aware! it can cause more damage than the promised benefits in the future.
Always stick to a balanced diet as the body requires all the nutrients.
Always include exercise whether it's weight loss or weight gain.
Always prefer natural ingredients over artificial ones.
Keep good hydration (your thirst should be the best indicator). When you are thirsty please drink plain and clean water and nothing else. An average of 8-10 glasses of water would be sufficient. If you are active and do workouts (exercise) then you can increase a few more glasses of water.
Avoid all junk food, fried food, very salty food, ready-made juices, sodas (cold drinks) tobacco and alcohol.
Eat freshly prepared food only. Have green salads, sprouts, whole grains (all types of grains), pulses, seeds, nuts, fruits and vegetables so that there is a good portion of all the nutrients in your diet.
Keep a track of your progress. Keep checking the vital parameters once in a week so that you are safe and healthy. And enjoy the process of transformation which should be slow and study. You should remember that it's not a competition with anybody but a conscious decision to stay fit and healthy which should make you happy and motivated.