Fattening others

You probably can't. Most people do not want to be fat. We kind of lose that sense here being with a lot of feedees.

Take her out to eat a lot. Always order fries. And always order desert.

The question was not how to gain weight, it was how to convince her to gain weight. We all know that getting someone to gain weight without their knowledge is wrong.

Yes.

Secretly adding weight gain suppliments to someone's food would be getting them to gain weight without their knowlege. This of course would be wrong. Offering them up large portions or extra servings is simply giving them choice.

It might be a subtle way of showing that you're ok with them putting on weight, a gentle way of letting them know, without freaking them out with a full on discussion when they're a bit unsure about themselves.

Did you ever give her even some small hints that you find her beautiful and don't necessarily want her to get even thinner?
Did you ever give her some small hints that if she gained weight you would still find her attractive?
Did you ever give her some small hints that it's not only thin girls you could be attracted to, and there have been girls larger than here before her who you liked?

How did she react?

From my experience, the best way to get anybody fat is letting them know how much you like the way they look.

The secret is to never mention her being fat, not even beautifully fat. Just tell her how beautiful and hot she is even if she doubts it.