I've just started learning myself and also use a G27 set up.
I play with a lower FFB for drifting and put it on 5 (I race with 7 set). Else, I find, it's too brutal and I can't be smooth enough with my steering control, which is pre-requisite.
Only after a bit of watching various youtube vids, reading tips (thanks to all who have offered advice on this site), etc... and a fair few hours of practice, did I start to get it. A controller is a joke compared to a wheel and anyone who says that a wheel will be easier is clearly lying, IMO.
Throttle balance is key to maintain a drift, as well as smooth steering control. Too much (throttle) and you will spin, not enough and you will jerk back violently and spin the original way. Practice to see what amount of throttle your car needs to stay in that drift and don't just let off the gas, the second your car feels like it's going sideways. If you drift too far to the outside a quick tap on the clutch should bring you back in. When you arrive at the point you wish to exit, let off on the gas a little. All throttle work should be smooth.
These above things have allowed me to (finally) pull off some very average drifts. More practice will only help
I hope this helps. Like I said, I'm a complete beginner (and drift like one) but at least I can finally see some progress, after following these tips.
Good luck.