Ok, I'm going to assume that you've already read Part 1. If that is so, you're going to love this because most of the previous info applies. Instead of downloading Wubi, create a USB Startup Disk. Wubi will be on the drive. Once this is completed, the USB drive can be used to do the wubi install described in Part 1 with one significant difference; you will not need to download all the files as they will be on the USB thumb drive already.

Creating the USB Startup Drive is a very simple procedure by itself:

Launch the USB Startup Disk creator tool from System->Administration->Create a USB startup disk.

Either insert your Ubuntu CD, or click Other and browse to your ISO file. (If the application doesn’t recognize your CD, try clicking Other and then Cancel. This caused my CD to show up.)
Plug in your USB drive or card. The application should recognize the drive immediately and check that there is enough free space. If you have multiple USB drives, select the one you want to use from the list.

Finally, you can choose whether you want your USB system to be persistent between boots, or static like a live CD. Adjust the slider to choose how much space Ubuntu will have on the disk to expand to, or select the Discarded on shutdown option.

Click Make Startup Disk, and wait while the USB system is created. Now you can boot from this drive on any system which supports USB boots.

Booting from the USB drive is just like a CD; you’ll have to select Try Ubuntu from the boot menu to load the desktop. If you allocated space for a persistent system, anything you install or change on the system will persist the next time you boot.