Normally such clauses should be there in the appointment order/offer letter itself. In the absence of notice period clause in the appointment order, the same can be introduced only by sending a circular/ notice to all employees as prescribed under section 9 A of the Industrial Disputes Act. In respect of managers you can make it even without any official communication but giving a letter to the concerned employee stating that hence forth your notice period will be one month/two months/three months but in respect of others who do not have any managerial powers, the notice period like any change in the service conditions would require common notice as per law.
The most important thing to be noted is that in order to terminate an employee the employer is expected to give notice but no where in any Labour Act it is mentioned that an employee should give notice if he wants to leave a company! That is why the fourth schedule to ID Act which describes the changes in service conditions requiring notice does not contain notice period. But in practice notice period has been recognised as one of the service conditions. Therefore we can try that. Otherwise,you may have to get a Standing Order drafted and certified by the labour Commissioner. Therefore, if the workers object to the notice (to be issued under section 9A of the ID Act) then you will have to drop the matter.
The format as per Industrial Disputes (Central) Rules is attached.