Gaining Weight

Does anyone have any tips on how I can gain weight? I gained around 20lbs total in 2018, but I’ve been feeling a little smaller now. My butt seems like its gotten smaller and I’m not sure why? Also my thighs seem to have gotten smaller. If anyone knows maybe why this might have happened & how to gain weight back & just in general, I would really appreciate it 💘💘💖💗 thank you 💓💗