Step 1) Format USB drive with fat32
sudo fdisk -l
(Shows device names. Note the name of the USB drive.)
sudo umount /dev/sd[?]
(This unmounts the drive so it can be formatted.)
sudo mkdosfs -F 32 -I /dev/sd[?]
(This formats the drive.)
Step 2) Write the ISO to the USB drive
cd Downloads (or wherever the ISO file is)
sudo dd if=ubuntu.iso of=/dev/sd[?]
(This extracts the ISO to the USB drive.)