copy my itunes music to mp3 format

I want to copy my itunes music to mp3 format and then onto a usb/flash drive so that I can listen to my music on my car stereo and so that I can transfer my kids music to their leap-pads for them to enjoy.

I am using a windows 7 computer.

How do I do this?

I did it once, but can't remember how so my usb already has my previous itunes library on it.

I have bought more music on itunes and want to update my music library on my usb drive.

I am worried I will delete files and be unable to retrieve them.

Thank you in advance for any help given! 🙂