When previous conversations with a well-trained employee---one who knows what is expected and has no obstacles preventing him from carrying out his work responsibilities---have not resulted in the desired work behaviour or performance, a verbal warning is the next logical step. A verbal warning is a form a disciplinary action that is issued by verbally communicating your expectations to an employee and warning him that you will take further disciplinary action if his performance does not improve.
Collaborate with someone in the human resources department of your office or speak with your immediate supervisor to find out the proper procedure for issuing a verbal warning. For example, even though you will speak to the employee, you will still need to document the warning in writing so there is a record.
Review any notes you may have made in the past regarding conversations with the employee regarding the same issues that have led you to decide to enact a verbal warning. Gather the facts of the situation before confronting the employee so your purpose and warning will be clear. Knowing the facts will also help you defend your position if the employee tries to argue with you about past events. Take note of anything you previously suggested that the employee should do to improve his behaviour or work performance. Plan to discuss why the strategies didn't work and led to the verbal warning.
Ask another supervisor to witness the verbal warning. Ask the employee to accompany you a private area or office. Explain the problems with his work behaviour or performance in a polite, professional manner. Tell him how he can correct the problem. Ask him if he understands how to correct the problem.
Inform him that you are issuing him a verbal warning. Tell him if he doesn't take the recommended steps to correct the problem, it will lead to further disciplinary action, up to and including the loss of his job. Ask him if he understands that his job is in jeopardy.
Tell him that you are confident that he can make the changes necessary to improve his work behaviour or performance. Be firm but polite at all times.
Document the details of the verbal warning in writing for your records. Sign and date the document. Ask the witnessing supervisor to sign and date the document. Do not ask the employee to sign and date the document. This is a verbal warning, not a written warning. Verbal warnings preceded written ones. If you have the employee sign, he could later say he never received a verbal warning, which could potentially cause problems during a lawsuit.
Even during casual conversations with the employee about work performance issues, always take notes to refer to later. Make sure the employee understands that you are issuing a verbal warning. Use the words "verbal warning" and ensure that he knows his job is at risk unless he takes the appropriate steps to correct the problem.
Listen to the employee's explanation when you issue the verbal warning. You may realise that there is a problem that needs to be resolved to assist the employee. Do whatever you can---within reason---to help the employee.