They will preapprove you for a certain amount. Then, based on the year of the vehicle, they will assign an APR % (the older the vehicle, the higher the rate). When you do the application, you tell Cap 1 whether you want to buy from a dealer or person and they structure the loan accordingly.
Once approved, they will send you a blank check (looks like it's from your checkbook with your name/address) but it's drawn on Capital One Bank. Most dealers are familiar with this process and shouldn't give you any trouble at all! As long as you don't go over your preapproved amount, you're good to go!
Good luck.