Road-head requires natural talent and acquired skills. You have to master the act of giving amazing blow jobs first. Everyone has to master the bike with training wheels on before taking on the real challenge.
My opinion, the perfect blow job uses both the mouth and hands. I usually deep throat while using my hands to play with the balls. I can't stress this enough. Don't ignore the balls. They don't just go away because you don't pay attention to them. They are there, begging to get some action too.
Deep throating is another challenge I think every woman should conquer. The less handling the cock requires, the hotter the blow job. If he wanted a hand job, he'd have asked for one. But no, he wants ORAL. For everyday blow jobs, deep throating is amazing but when we talk of road head, remember roads aren't always flat and you need to know how to take a dick in your throat in the event of a ditch or other unsavory road bumps. I had trouble in the beginning thinking I had to slide the cock slowly down as far as it would go. And I'd gag. Then I started opening my mouth and taking as much of it in my mouth as I could before closing my mouth. It works wonders. It is almost like bypassing the soft palate completely therefore eliminating the gag reflex. I've tested this on a range of sizes and it works with them all.
Now you're ready to get down to business in your automobile. I suggest rubbing the cock until it becomes hard before you begin. If you want an audience, get in a sedan or sports car. If you're more of a shy person, you better have an SUV or truck. The higher you are, the less people will see you. Night is even better if you're looking for a more private experience. Be aware of the front seat. The easiest way is when there is no hump or center console but that is rare now a days. So you're going to have to figure out how to situate your body across a stick shift or center console. I usually unbuckle my seat belt and lay my stomach on the console. I have more control this way, using my abdominal area to keep me steady. Once the cock is out, reach your hands down to the balls and take that sucker in your mouth. Do it however you like. Make it slow and passionate or wet and sloppy. Don't let your mind wander into thinking "oh no someone will see" or thinking it is somehow different from how you normally give a BJ. It's not. Location change? Yes. But a cock is still a cock and your mouth needs to be put to good use.