The only thing I can suggest for sure is: Practice
There are many ways to start a drift that come in my mind (like using e-brake, using weight shift (ie. first go the wrong direction of the turn and then suddenly switch to the way you want to go), setting the brakes more to the rear of the car, using the dirt to lose the grip of the rear wheels, shifting down, just letting the acceleration go and turn in, etc.)
But everything just listed is a matter of practice and trying stuff out.
It as well can be of help just to spectate other players that are doing alright.
So, just keep on trying and don't be disappointed if you're not able to achieve a good way of starting a drift very soon
