It takes a lot of practice. It's not so much about flooring it until the rear-end kicks out, because if you wait for that to happen then it's already too late. You'd probably realize this intuitively if you did it enough, but hopefully I can give you a leg up on it.
Drifting is a game of prediction. Actions you take won't necessarily play out immediately, so you need to be able to predict the car's behavior and often respond to situations before they even arise. The weight of the car takes time to shift from one wheel to another, it's not instantaneous. If you think of a kart or F1 car, they are so rigid that they respond almost instantly to any driver input. The kind of cars you would drift aren't that rigid. Fortunately, the car gives you subtle hints that you'll learn to pick up on that will tell you what it's about to do. Keep that in mind and you should improve.
