Thicker then a snickers

I see all these post about losing weight, well im going on 26yrs old and i cant get past 120lbs unless pregnant and even with both my babies i went no further then 135 with each of them and lost it all back. Can some one who has purposely gained weight give me any advice on how i can put on atleast about 20 pounds lol?? My weight goes up and down i need to gain and keep it!