How to gain weight?

Brooklyn β€’ πŸ”žπŸ‘ΆπŸΎ
I went to the doctors office today and they said I need to gain at least 20 pounds before the end of my pregnancy. I'm 18 weeks a weigh 108 pounds and I'm 5'6 so that's understandable but I'm not sure how to gain weight. I've always tried even before my pregnancy with no success!