There are several types of handover letters. For instance you can write a handover letter to your employer, your fellow employees or directly to costumers, to let them know who will be taking care of their requests after you leave and how to contact the new person in charge of that role.
However, no matter which type of handover letter you are trying to write, its main purpose is to help your successor perform their new role confidently and effectively.
Also it should help you to see how you have contributed to your job/team.
A good handover letter should always include:
1 - A detailed description of your job, its processes and procedures. All your tasks, duties or responsibilities need to be stated as clear as possible.
2 - Login details: Usernames and passwords needed to access any work related computers, software, email accounts etc.
3 - Opening/closing duties with a step by step description of all the tasks one needs to perform for a seamless shift transition.
4 - A list of all current projects and their progress statuses. You should also prioritize the most urgent ones.
5 - The places one can find the keys for any cupboards/drawers containing vital documents and where they should be kept.
6 - Any other information you think is relevant for your successor to know about.