follow the following steps and you should be able to gain weight
1.eat 5 to 6 meals a day. you should eat at regular intervals, and even if you are not hungry or anything still eat something, that way your appetite will increase naturally.
2. work out . full body work out atleast 4 times in a week is necessary.
3. have about 600 calories more than usual. you could do this by having a milk or protein shake or juice or a fruit at regular interval.
4. get good sleep. atleast 7 to 8 hours of sleep is necessary or else that can cause a lot of problems.
5. make proteins a part of your diet rather than going for high fat diet.
6. be consistent and don't lose hope because weight gaining is just like weight losing, both require persistence. with all the above stated advise you may be able to gain atleast 1 to 2 kgs in a month. but that's better than not gaining right?
and I suggest you take it slow only because faster methods could be harmful
for more information you could visit a dietician.
all the best.