How to gain large amounts of weight in a shortish period of time?

HashtagFaceGram
Two things to keep in mind when answering this question. First and foremost, I don't want to do anything that will make me vomit, isn't physically possible, and cannot be sustained. Second, and equally if not more important, I am not trying to gain weight in fat. So the first person that says "Eat five brownie sundaes"-A. Fudge you- B, you're getting a kick to the brownie. I see no possible reason to gain a bunch of pounds of fat. I know that you can gain fat, but I would rather gain the weight in muscle. I want to gain a good amount of muscle by the summer. Meeting the criteria I listed above, how can I achieve this. (I picked summer because it is one of the main blocks of time, plus it would be good to feel comfortable in a bathing suit which I'm not because I'm so skinny.)
How to gain large amounts of weight in a shortish period of time?
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