Hi,
An exit policy is nothing but processes and procedures to be followed once an employee submit resignation or when the services of an employee is terminated.
So based on your Company process you need to prepare the Exit Policy.
For example : Once an employee submits resignations the initial process is his/her immediate superior will have the discussion. If the immediate superior is not able to convince the employee then he need to escalate the same to the Head of Department / HR. What are the steps to follow once resignation is confirmed by the employee even after repeated discussion etc..
So you can try and develop a exit policy based on your company process and submit here if any correction is required.