If you do want to go the Ferber route (which is what I think your question was), you go in after increasingly longer intervals. 1 minute, then 5 minutes, then 10 minutes, then 15, you get it. Speak in a calm voice, you can pat her back, but don't pick her up.
Ferber worked with our first (although we did it around 18 months, so she could understand what we were saying to her) and we didn't need it for our second. It really does work with most, but not all, kids, but you have to be willing to commit to a couple of weeks of unfun. It doesn't take most kids that long, but it can. It's bad the first night, worse the second, and better the third and beyond.
As far as CIO being cruel, I don't think it is. What I do think is unfair to a kid is to keep changing the rules on them. So you need to decide if you can tough it out through the passionate crying. If you can, I bet it will help your daughter sleep without you. If you can't, try another method, like co-sleeping. I always felt I was a better mom if I was reasonably rested, so more harm would come from years of exhaustion. Your choice.
Good luck. Sleep issues suck.