A hard gainer is a skinny person (usually male) that has an extra hard time gaining any sort of weight. It seems as though they eat and eat only to find that their weight stays the same. Hard gainers are still human and because of that, gaining weight requires the same steps that everyone else needs to take. Hard gainers simply need to work harder because of their fast metabolisms.
1. Eat More. Creating a calorie surplus is the most important step in gaining weight. If you're not eating enough calories, you might be in a calorie deficit which will force your body to burn fat stores and even muscle tissue for energy causing weight loss. Muscle growth is a very energy intensive process, if you're working out and not gaining any weight, you're probably not eating enough. Gaining muscle isn't only about eating a lot, it's about eating a lot of the right foods. Gorging on donuts and Big Macs will lead to weight gain, but it will also lead to excessive fat buildup which is unsightly and unhealthy. You don't want to be fat, you want to be muscular.
2. Lift Weights/Strength Training. Simply eating enough isn't enough. Having a calorie surplus will force your body to put on weight but it won't necessarily be the kind of weight you're looking to put on. Your muscles will only get bigger if you stimulate them. If you sit around your house eating a lot and watching TV, you'll get fat rather than muscular. Getting bigger requires 3-4 strength training sessions per week. Strength training can be lifting weights or engaging in body weight exercises such as squats or push ups. Though you don't need a gym or weights to get bigger, having access to equipment will give you better and faster results than using body weight exercises alone.
3. Supplements. At the very least, hard gainers should be taking protein powder in the form of a post workout shake. During am intense workout, you create tiny tears in muscle tissue that is repaired during rest. It's during rest that you actually get bigger and stronger. For your body to properly repair damaged muscle tissue, it needs the proper nutrients. A post workout protein shake that contains an easily digestible protein (whey) and a simple sugar (ice cream, honey, mixed frozen/fresh fruit) will help kick start the recovery process. Other supplements that can help you reach your goals include a multivitamin, BCAAs and creatine.
4. Count Calories. The main rule of gaining weight is eating more. Sometimes mentally keeping track of your calories won't give you an accurate picture. A reliable fix for this is counting your calories. Though this may seem daunting, it will help ensure that you're accurately counting your calories and portion sizes. Too often, when you're not gaining weight, the problem is that you're eating less than you think you are. After you start counting your calories, if you still aren't gaining any weight, increase your intake by 200-500 calories per day and re-evaluate your needs after a week or two. Continue this process until you start gaining weight. Use the calorie calculator to figure out how much you're supposed to be eating each day.
5. Less Cardio. Cardio is a type of workout that burns large amounts of energy/calories. It promotes endurance, helps with weight loss and strengthens the cardiovascular system. Because of its health benefits, cardio should not be completely ignored by anyone. If you're a hard gainer however, consider reducing the amount of cardio you do each week and see if it helps you gain weight. Another (probably better) option to consider is to offset the amount of cardio you do by eating more. If you burn 200 calories during a cardio session, eat an additional 200+ calories that day.