Step 1. Find a person that wants bitcoin AND has a car that they are willing to exchange to get bitcoin.
Step 2. Offer that person bitcoin in exchange for the car.
Step 3. Negotiate an amount of bitcoin to be exchanged that is agreeable to both you and them.
Step 4. Exchange bitcoin for the car.
There you go.
Or better go to Bitpremier and select from the listings there. However, I must warn you that on average, the listings there are at least 10% to 15% more expensive than the actual prices. Also, I am not sure about the tax implications if you use BTC to purchase an automobile.
https://www.bitpremier.com/2-autos