Easy enough, you need more than just navtweak,

You need ,

Navtweak,
GTK+/Win32 Runtime Environment files
An image reader/writer, (like imgburn)
An application that can read/write and open and image, (like UltraIso)

Once they have installed all the above, you then download the OS image, open with UltraIso, drag and drop the navboot file from the correct dir in the image file to the desktop, mod navboot and save, drag and drop back into the image, save, then burn new image (with imgburn) at a slow speed and job done.

It sounds hard but once you have done it few time it's second nature.

__________________
Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.