Ditzy:
Like others have said just start eating more at meal time and snacking when you can.
The is no magic pill or food or method that will make you fat that is up to you and you have to achieve it on your own.
Its not as easy as you think it might be you really have to work at it and it takes time.
It took me close to 8 years to go from 115 to 300 lbs.
Sooo hot to triple ones weight :-) My wife’s did the same, though she’s been north of 300 for about 10 years now.
As for a gaining formula, you can’t go wrong with the 3 P’s — pasta, pizza and potatoes!