I wanted to increase corroption and lower willpower.
Turn on Dominate and Allow Sex.
Increasing corruption requires for the trainer to have 70 corruption. Turn on any 1 template, such as Mental.
The base skill is calculated mostly the same as Slave Master/Mistress in the wiki, only there's an adjusted bonus for high fame (number of combats fought) and a new value for affection towards the MC (-5 below neutral, 0 at neutral and +5 at Soulmate).
The educator will also by default train discipline up to his/her own value, max 100.
Trying to train non-slaves can get a little tricky, but it's explained well in the Allow Sex tooltip.