Try to follow these weight gain diet tips and the result will start showing within 7 days:
Protein forms the building blocks of your muscles. Eating sufficient protein is required to gain muscle weight instead of just fat. If you're trying to gain weight, aim for 0.7-1 grams of protein per pound of body weight (1.5 - 2.2 grams of protein per kilogram).
Eat at least 3 meals per day and make sure to eat plenty of fat and carb.
Eat a lot of energy-dense food. Add a lot of spices, condiments, and sauces so that the food becomes tasty and you can take more of it.
Lift heavy weights and improve your strength. This will help you gain muscle mass instead of just fat.
Squeeze in an additional meal or snack whenever you can, such as before bed.