Plug your USB Flash drive into your computer's "USB" port. The drivers will take a second to load, but you should see a message on the bottom toolbar indicating that "This device is ready to use."
Go to "Start" on the left end of the toolbar and click on it. Go to "My Computer." Then click on the USB Flash Drive icon on that screen. This will give you access to see what is contained on the drive already, and what is being put on it.
Drag the audio files that you want to listen to, into the folder for the USB Flash drive screen that you opened in step 2. When you are satisfied with the amount of audio, you can safely remove the hardware by right-clicking and selecting "Safely Remove Hardware."
Insert the USB Flash drive into the car stereo's USB port. Select the appropriate input from the stereo's menu. Now you can listen to the audio that you transferred from your computer to the USB Flash drive any time that you would like.