"I don't have as much experience with this, but I've got some. ... Which is probably more than most holocrons, so go me, I guess. I think that what redeemed Revan wasn't the brainwashing, it was - reminding them of what they were fighting for. And, admittedly, putting them in a situation where what they thought they wanted wasn't achievable anymore. More telling is when they helped redeem Bastila Shan. Which uh, mostly consisted of them beating the ex-Jedi over the head to knock them out, then throwing them to the Jedi Council to fix them. They weren't involved in that one, but I bet there's records of it somewhere.
"So since I lack in a lot of personal experience here... I think the key is to remember that the dark siders are all kind of, uh. Traumatized. Traumatized is the word I want to go for. And people handle trauma very, very differently. Maybe see what Sith do when you take away their ordinary methods of torture, intimidation, and murder? Once those are out a Sith is starting to look a bit bereft of choices unless they're one of the smart ones. ... Look for the smart ones." Pause. "I'm really not as good at this one. Talking to people and being there for them is about all I've got, sorry."