When I had anorexia, I weighed 92 lbs and I was told by my doctor I had to eat in order to live.
He placed me on weight watchers and told me to eat everything on the pregnancy menu and to drink protein drinks in between meals three to four times a day.
I ate butter, chocolate, meats with a higher fat content such as beef and pork, also lots of nuts and I had eggs three times a week and drank whole milk every day.
Of course I did not stay on this diet or I would have been at 200 lbs, but the doctor had me stay on it for 5 months and it worked.
If you have lost weight due to cancer treatment like my grandmother or have anorexia or other, I would recommend you stay under a doctors care so he can monitor you and to make sure your weight returns to normal.
The best way to gain weight is to eat more calories than you burn. Eat foods that will help you gain slant mass, not fat and to do this, you can have six meals a day, that is, breakfast, lunch, dinner and three other small meals scheduled throughout the day. You can eat food like chicken, fruits, milk, vegetables, steaks and cheese.