Advice on drafting a resignation letter?

On your resignation letter, explain that you would be starting in two weeks time and tell your employer about the cirstances. Knowing that your job is indeed vital for your employer makes him hold on to you considering also that you have been employed for 5 years. That is why he needs you to train the replacement for quite long but just remind him of the employment agreement in your resignation letter. Nothing would be wrong though if all would be made in a polite way. I am certain he will understand you.