If you need to write a letter of permission for leave, explain clearly but concisely why you need to take off from work (e.g., to deal with a medical issue). You might also suggest how your work will be completed while you’re away—for example, you might arrange for someone to cover for you, or get some of your work done ahead of time to prevent a buildup. Provide specific dates so your boss will know how long you’ll be gone and when you expect to return.