How do I healthily gain body fat with a high metabolism?

Why? Why would you want to gain body fat? Suggest you stick to a healthy diet balnced in protein, carbs, and fat. Exercise daily. Trying to gain fat is like to tryin gto lose weight by starting smoking.
Follow healthy diet. This issue is over rated. As a general rule as we get older and with pregnancy, people will gain weight. I do not agree with any mode or food/ pills to cause weight gain in general, because all their long term consequences are not good.