Follow the steps below to make a flash drive bootable on ubuntu
Step 1: Click on the ubuntu search and type startup Disk like on the image above.
Step 2: Click on the startup disk and a window will pop up (see image below), click on other to select your ISO File, then select from the box below the Disk or flash drive to use for this purpose.
Step 3: Once step two is completed click on Make Start Up Disk and wait for few minutes for the image to be burned into the flash disk.
Step 4: You flash is ready. Have fun.
Do let us know in the comment below if this did help you (Note: This process only works for ubuntu ISO Files. if you want to make windows bootable on a flash disk on ubuntu click here)