How do you write casual leave letter?

Requirements for requesting time off under a casual leave policy will vary by company. However, in most companies there is no need to write a letter. An email will be adequate. In the email, typically, you can address your manager the way you normally do in conversation, tell your manager that you need some times off, exactly when the time off will be, and request permission. Depending on the company and the manager, you might also want to give a reason that you need it, and refrain from making plans before you receive permission. In some places, the time off is almost a given, but in other situations it is very dependent on the needs of the department at the time.