Give your name and any affiliations you may have (e.g. member of a spotter organization, amateur radio club, sheriff's department, etc).
Provide a detailed description of what happened, including any damage, measurements, injuries, or fatalities. Mention how you arrived at your values. For instance, don't just say there was "nickel size hail", say whether you measured this with a ruler or estimated.
Try to give as precise of a time as possible for the event occurence.
Give as exact of a location as you can when reporting. Try to give a distance to the nearest tenth of a mile from the nearest town, village, or city. Cross streets can be very valuable to include in your report, especially if you are in an urban area. The bottom line is be aware of where you are! Also, be sure to distinguish between where YOU are and where the EVENT is occurring.
EXAMPLE OF A GOOD REPORT
"My name is Joe Public, and I'm a trained storm spotter. I measured quarter sized hail with a ruler at 5:30 p.m. about 1.5 miles west of Amarillo. Also, at the same time I can see a rotating wall cloud about 2 miles to the west of my location."