This should get you started:
Follow the video link. Buy a trainer (Keen Edge Knives makes a pretty decent aluminum trainer). Practice a lot and then start some sparring drills with a partner. Get some personal instruction if possible. If you are ever in Ohio, look me up and I'll give you some pointers. I do not recommend trying to learn from video, and see video best used as a reference for those who have some training. However, aside from the link I posted to my website, I would suggest you look up Ray Dionaldo of FCS Kali (he has a few good videos out on the web and I believe a DVD or two for sale). Tuhon Ray is very good with a karambit and, if you get a chance to go to one of his seminars or training sessions, I would highly recommend it.
Good luck with your training,