What is your current weight and how much are you looking to gain? And are you looking to gain in purely with food or do you plan on doing exercises?
You can increase your weight like anyone else actually by having a surplus of calories at the end of the day. First and foremost your blood glucose needs to be under control then when you start increasing your food intake you'll most likely need to increase your insulin too. Diet is easier when you have a certain routine as it's more clear how much calories you're getting and how much extra insulin you need. Obviously it's not advisable to do a dirty bulk, your best option is high calorie "clean" foods (i prefer nuts myself). Others foods are fat fish, avocado, olive oil, turkey, chickpeas, ... list goes on. Some cheeses are good too. One of my favorites is filling a portobello with goat cheese and nuts crumble then sticking it in the oven (an easy 1000 calories there).
Furthermore several meals a day with snack in between are more advisable than eating all those calories in 3 or 4 servings.