What format should I use?

The format you choose is dictated by the audience and how they will consume the documentation.

It should have the following components

• Use case
• Steps to complete the task
• Who requested the functionality
• Why the functionality was requested
• Date & time the functionality went live

Make your documentation easy to scan. Tables and bullets are a great way to highlight the important bits.

Revise relentlessly! Keep a revision history so you know who made changes and when.