» Make sure you have atleast 2 GB space in your USB stick .
» Download a fresh copy of ubuntu 12.04 iso (http://www.ubuntu.com/download/desktop)
Creating Ubuntu bootable USB stick on windows :
Windows users have to Download USB installer (http://www.pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/) . USB installer is to put ubuntu 12.04 into the USB Stick as bootable image.
under step 2 » press “Browse” to select the Ubuntu Iso file download.
under step 3 » Choose USB stick Drive and Press “Create” .
» Press “Yes” to continue
» Press “Close”
That’s it , You have created Ubuntu 12.04 Bootable USB on windows.
Creating Ubuntu bootable USB stick on Ubuntu:
» Open “Startup Disk Creator”
* Classic Desktop Users » Systems –> Administration –> Startup Disk Creator .
* Untity Desktop Users » Just type “Startup Disk Creator” in the Search .
» Press “Quit”.
Now you can boot with this USB stick for installing Ubuntu 12.04 .
Boot with your USB drive and follow the post Ubuntu 12.04 installation step by step screenshots