I think the best way to do it is by using the launch control feature that is on all cars. Here's how you do it:
1.) Put it in 1st and set the E-brake.
2.) Apply 25% throttle. The revs will rise and stabilize. Don't have any more input as the E-brake will release.
3.) When ready or at the start of the race, mash the gas and watch it go!
Alternatively with higher HP cars, you can launch in 2nd or even 3rd gear is have the gearing correctly. You can also make a nice smokey burnout with that method. An example would be a stock Ferrari 458. You can launch it in 2nd gear just by holding the E-brake, revving the living snot out of it (or, full throttle), and just simply release the E-brake.