Write down the driver's license tag number, as well as a description of their vehicle. Be sure to adhere to driver safety rules and keep a close enough distance from the driver's erratic driving while getting this information.
Write down the present location of the driving offense, as well as the general direction the driver is headed. Information such as the street or highway you are driving on, or intersection you are close by, can give the authorities a clearer idea of where to find the offender. If you see the driver turn down a different street, or exit the highway, write down which way you see him driving toward.
Call your local police department and provide them with the information over the phone. Give them all of the details that you have written down and notify them that you want to file an official report. Depending on the nature of the driving offense, especially if a hit and run is involved, you may be asked by the authorities to fill out a detailed report when they arrive.