What's the best method to gain weight?

Below. If you consume more calories than you expend during the day, you will gain wt. Any online calorie counting site will give you information about the calories in food.
Wt gain. Good well-rounded diet with an increase in your total calorie intake.
Eat, eat, eat. You can gain calories by just eating more food. People tend to gain weight with meat because it's more calorie-dense than other foods, but you'd gain more weight by just eating more of less calorie-dense foods. Just keep it healthy - there's a difference between gaining muscle weight and just getting fat. And exercise as well, so you gain lean mass and not just get fat.