Help with Flashnul

[Hoags]

Hello all.

Please excuse that I am going to ask will probably sound stupid.

Here goes. I am trying to load ubuntu onto a usb drive to give it a go on my friends laptop. I can get it onto the drive and do the image thing, but I cannot do the flashnul bit to finish it off. Can someone please give me step by step instruction on how to carry out that bit, I am not a computer wiz and dont understand what it wants, or how to do it?

Thanks in advance.

Hoags