1. Date, time and location of experience.
2. Number of people involved in the sighting. (Ages, genders, occupations as well.)
3. Detailed description of experience. (Position, movement, colors, smells, sounds, physical feelings, etc...)
4. Any after effects. (Psychological, physiological, emotional).
5. Any physical elements potentially able to be detected.
Further useful details would be whether the case has been investigated prior to submission, and contact information for others involved in the sighting.
For further reading, see John Keel’s “The Subject of Saucers” from Anomaly Magazine #1, May 1969.