Installing Ubuntu to a Flash Drive via Startup Disk Creator:
Note: Back up ALL data from your Flash Drive before proceeding!
Insert your Ubuntu CD (or if you have iso file than dont do this step) and restart your computer, booting from the Live CD
Insert a 1GB or larger USB flash drive
Navigate to System > Administration > Startup Disk Creator: Start Ubuntu's Startup Disk Creator
if you have iso file of Ubuntu than select from source disk iso file. than further proceed.
(1) Select your Flash Drive from Disk to use
(2) Choose to Erase Disk (Make sure you have backed up any important data first) Erase Disk (Make sure you have data backed up first)
(1) Select the partition related to your Flash Drive,
(2) For Persistence, select the option Stored in reserved extra space and adjust the slider to desired capacity
(4) Click the Make Startup Disk button: Proceed to Make an Ubuntu Startup Disk
A bar appears to indicate the progress of the install. Once the installation is complete, remove the CD, restart your computer and set your Boot Menu or System BIOS to boot from your USB device.
You should now be booting from your Ubuntu Startup Disk
I hope this will help you.