As there are detailled descriptions on how to install Ubuntu on an USB stick (aka “pen drive” or “flash drive”) they assume you have a running Ubuntu System.

To install a CD installation image on an USB stick with any Linux distro, just use dd to put the *.img file on the sticks device (i.e. not on a partition).

dd if=/path/to/ubuntu-X.XX.X-Arch.img of=/dev/sdX bs=1M