Seems to be simple enough. The open changes nothing (as far as I can tell) and after that you have to train the slavegirl, by proxy.
Green bar is hp, if it runs out, she faints. Presumably this leads to something bad.
Blue bar seems to be her arousal. Get it to 100, or whatever.
I'm testing out cheat-engine to see if i can lock her hp at 100, just to make things easy for me, but it doesn't seem to be happening.
TLDR: train her so her sex-stats go up (including her max hp) without breaking her.
As for controls... just faff about with the buttons till you get the gist, basically.
3rd process, value * 2 for CE :v
Just know that while the number will still visually go down when you do the next action, behind the scene it's still locked.
(for example, say I lock her health at 70, do an action that drops it by 8, the screen will still show 62, but when I do an action again it'll calculate based on her health being 70)
For her arousal, getting it to 70+ allows you to choose to make her cum, doing so gives you an elf's tear item for crafting, as well as usually being one of the better health:stat gain option (of course if you CE anything is fine)
You unlock more training methods based on the girl's stat growth, though some requires you to send the dark elf to go hunting and gathering materials for crafting them (like BDSM rope or whip)