Cool that you're interested in gaining.
>idk how to start
Depends. The easiest way is probably to eat highly caloric food, at large quantities, and to be a lazy fuck (so that you don't burn any of those calories), but that's also not very healthy for the long term. Ask yourself if you want to gain as quickly as possible or if you're okay with a bit of a lower pace (also, not everyone gains at the same speed; if you have higher metabolism, you might need to increase your calorific intake). If it's a fetish thing, think about the kind of gains that turn you on (some people like to be lazy, some prefer gaining and staying relatively fit, some like to just try and make themselves as unhealthy and out of shape as possible). What's your diet, lifestyle, exercise etc. like now?
As to when to stop -- what kind of body would you like to see on yourself?