Select the USB stick device (you may need to enable the option View -> Show All Devices) and select Erase from the toolbar (or right-click menu). Insert your USB stick and observe the new device added to Disk Utility.
Launch Disk Utility from Applications -> Utilities or Spotlight search. To ensure maximum compatibility with Apple hardware, first blank and reformat the USB stick using Apple's 'Disk Utility', but this step can be skipped if you intend to use the USB stick with only generic PC hardware.
