The starting line was a line painted on a street that led up to Courthouse Square. Doc Brown positioned it at the Bluebird Motel about a mile from the courthouse and instructed Marty McFly to drive to that point in the DeLorean, push a hook and rod into the flux capacitor and wait for the alarm on his dashboard to go off.
As Marty knew he was in a time machine, he reset the time circuits for ten minutes prior to Doc's death in 1985. The car's engine then died. Marty eventually started it up after the alarm went off and made it to the cable just as lightning struck the clock tower and Doc was able to re-connect the cable.
Behind the scenes
The real life filming location of the starting line was in Griffith Park.