Eat loads of sweets, crisps, and drink a lot of fizzy juice and sit on the couch watching T.V. all day, keep doing this for 45 days, then u will be fat. But make sure you don't eat too much salty and...
You really shouldn't try for a fast weight loss. What's put the weight on is a long period of poor lifestyle choices, and what'll give you the best shot at not only getting the weight off but also...
Twinkies, even better, deep fried twinkies. And if you're really desperate some drugstores sell nutrition bars that add weight onto your body. But I don't know why you'd want to be fat...