Gaining

Well caroline, you are not really fat ... yet ... just very plump and curvy!

Start drinking heavy cream in the morning ... say a half pint at first, with breakfast, and another half pint with cookies before bed. Work up to a pint in the morning and a pint before bed.

Keep it up for at least 3 months and as long as you desire after that.

I predict you will achieve the gain you desire.

The real trick to a continuous and healthy gain is to keep your body metabolism guessing. Have a wide variety of a diet as well as an expansive variety of activity. Body has an easier time and need to hold onto things if it doesn't expect whats going to happen.

Activity actually switches the body into absorbing what it gets rather than throw it all out when it in a long period of inactivity. Not saying go run a marathon or anything but, some exercise goes a long way in building both fat and a lil muscle.

Don't cut out nutritious veggies or fruit. This is a monumental mistake to do. Just add it in with the higher intake of calories giving you body reason to take in more nutrients. You feel better in the long run for it.

Try to be as inactive as possible for a month or so. This way your metabolism will slow down, your muscles will melt away (your body burns calories to keep muscles up), and you will become accustomed to a kinda lazier lifestyle. All of this, together with big amount of fatty food (stay full, be greedy and gluttonous) will make wonders.
Please keep us updated and good luck!