It's easy, find out what your matenience calories is (calories consumed = calories expended) add 500 on top of that per day and you will gain 1 to 2 pounds of fat a week. Gaining weight any faster than that is unhealthy to the body. Or you could just lift and eat a bulking diet. I thought I couldnt gain weight untill I did some research and I managed to pack on 17 pounds since last spring. Very little fat came with my bulk since I was lifting heavy.
We pay our debt sometimes.