I used to be a skinny person but needed and wanted to put on some weight too.
The way I gained weight was to make a weight gainer shake at home by mixing some oats, milk, bananas, peanut butter and whey protein in my blender.
You should drink at least 1 to 2 of these weight gainer shakes per day.
It took me around 4 months of drinking these shakes to start seeing some weight gain and around 6 months to gain the needed weight.
You can also increase your calorie intake to help you gain weight and eat high calorie snacks such as yogurt.
You can also buy weight gain powder and pre made weight gain shakes online which have good reviews although I never tried them.