Just in case you are unaware. You cannot simply give a written warning with no notice to an employee any more!
You have to give 24 hours written notice for any disciplinary action taken against an employee regardless of what type of warning is going to be given. Also the employee is supposed to be given the opportunity to have another person attend the meeting with them for support, although this person cannot contribute in any way. You are supposed to interview the employee and then take a break to decide on the outcome of the meeting. Then you can tell the employee what action will be taken, warning, dismissal etc.
If you do not follow this procedure then the employee will have grounds to bring action against you in the case of any dismissal and they will probably win!
In the case of the letter, you simply have to put what 'offence' they have committed, the outcome of the disciplinary meeting, what you expect from them re their behaviour in the future, what could happen if they do it again and the fact that the warning will remain on their records for a period of how many months you intend it to.
Interestingly, you do not have to follow any of this procedure if the employee has been with you less than a year and you have stated this fact in your contract! So with any employee that is underperforming and has been with me less than a year, it is easier to simply sack them on the spot with no prior warning, which seems very unfair (I don't do this often by the way!).
Hope this has helped, I am just trying to protect you from any possible comebacks.