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To gain weight, you have to eat more. This can be done with extra portions and calorie dense foods. Most people do best with three meals and at least two snacks. Don't go more than four hours without eating during the day. If you are working, this may be difficult, but you can plan ahead and bring your snacks with you. This is a good time to have meal replacement drinks or bars that don't require much preparation. You also have to do weight training. You can start by working out three days a week on alternate days, then go to a split routine, where you work each muscle group twice a week. Make sure you emphasize big muscle group exercises like squats or leg presses, bench presses, and pulldowns or rows. Supplements aren't really necessary, except for a multi-vitamin/mineral and the meal replacements, or protein supplements if you don't eat much meat.