Looks like you have the same problem as I did. Like everybody else has said, it's very difficult to teach tricking. Corks are very daunting, and they're difficult at first, but I found that after I landed my first, I could pull them off constantly. Setting up can be difficult. Just make sure you have your foot flat on the ground to give the most grip. When you begin the cork, you'll need everything to reach it's peak at the same moment. Your arms should be fully raised at the same time as your jumping hip is fully extended and your other is mostly contracted (not all the way, or it looks like you're eating your knee). At the same time, your arms should be raised, your pecs and chest spun, and your abs extended. This is some serious timing that is hard to perfect. Once you're in the air, it's usually best to go into a semi-balled position. Not all the way, or it looks bad, the more laid out the better. Lower your arms and half contract your elbows. Contract your knees just a tad, and bring them to a position that is not fully straight, but just a touch beyond. Keeping this form will make you keep good spin and rotation speed. This is how I do corks. However, don't get into a habit of doing corks all the time. Many trickers can do them, and too many in one replay without variation can get boring.

I hope these tips helped! Good luck