Taking a knife away or just stripping it from your attacker is a higher level of skill and relies on thousand upon thousand of repetitions. And even then it requires you to have a bit of luck on your side.
The golden rule is that you don't go in looking for a disarm. Disarms just happen. They come to you. Again this is a higher level of skill. Once you get good at these, you should then practice with a live blade to bring you back to reality!
Disarms should be practiced empty handed, when you're also holding a knife, when holding a stick/axe/machete, etc.
We learn the basic sets vs. heaven grip and earth grip. Here is a YouTube video showing the first 5 of 12 disarms from the heaven grip.