Take pictures. As soon as the crime scene is secure, and before investigators go poking around, document the scene at hand with your camera. Take multiple pictures of areas from different angles -- don't worry about overdoing it, the more pictures you have, the more complete your report will be.
Describe the scene into a digital or tape recorder as you take pictures. Your description will help you remember what each photo captures. Be as descriptive as possible. Describe how items are strewn about the room, and whether any blood is visible to the naked eye. Describe locks on the doors and whether any lights are on or off. Check the dates on any mail or newspapers. Describe how part of the scene is left untouched while another is completely in disarray. Be as vivid as you can; leave nothing out.
Write an outline of the scene on your notepad using your photos and voice recording as your guide. Make note of the most important parts of the crime scene, and write down any clues or leads you might see. Watch for patterns that might be consistent with other crimes you've investigated or read about to see if the method matches.
Write your report or story in full, clear, concise language. Begin with the most important information, clues and leads to make sure the reader understands the point of the story immediately.