The only tip is to gain weight. Eat richer foods. But richer, eh, not toxic. Nuts, potatoes, stuff that's calorie-high but still healthy and nourishing.
And carbs... not really. Go for proteins and fat. Fat, mainly. And unsaturated fats, mostly, because saturated and trans fats aren't any more caloric, but are more unhealthy. Carbs are okay, but don't target them specifically. Because a lot of carbs means you're weakening yourself to diabetes. And complex carbs (starch as in bread, potatoes and oatmeal) are better than simple carbs (sugar) because they make your insulin levels yoyo less quickly. Basically, I think if you follow the nutrition advice for bodybuilding but "forget" to do the exercise, you'll end up putting on fat.
Oh yeah, and I'm a vegetarian, but if you like meat, I think eating a lot of that is pretty effective. Get lean meat though. Animal fat is mostly evil.
And don't gain that weight super fast. I think when you gain weight in a short time, the body tends to deposit it all in a "dump" which might very well be your belly. I think it's best distributed if you gain it gradually.
You could also search for "foods to avoid on a diet" and eat those a lot. x3