Congratulations!
Eat any sweets, cheese, beef, ice cream, and drink soda regularly and eventually you’ll gain weight. Eat chinese takeout food, the fried dishes, at least once or twice a week. These dishes are very oily and fattening too. I’m not sure why you want to gain because you’re probably better off saying thin. It’s okay to be naturally thin and gain weight naturally if your metabolism slows down.
I recommend a slow steady gorging process combined with assal horizontology.