Eat a lot of healthy food. Get you a good combo of carbs and protein and work out regularly. Lifting is better since cardio makes you lose weight easier.
Or you could say screw it, laze around all day snaking and eat a lot before bed each night. Ice cream in particular. But I don't recommend that as it makes you rather pudgy. Plus you'll have no energy to do anything. But at least it'll give you a belly, if you're into that sort of thing.
1. Eat lots of Hot Pockets and McDonalds
2. Drink lots of soda
3. Under any circumstance DO NOT EXERCISE
4. Minimize walking, maximize couch use
5. No smoking or caffeine. These are appetite suppressants;
6. If legal, smoke weed then crush chips and dip. Repeat.
PB&Js every time all the time with lots of protein, though gaining weight through muscle mass is a better alternative. Just be aware if you bulk and then go skinny again, your skin will be stretched to hell.