Assuming you are using Windows.

1) Install the Nokia Flashing Software.
2) Plug your n800 into the computer via USB (leave it turned off for now).
3) Run the Nokia Flashing software
4) When it gets to the point where you have to turn on your tablet and hold the home key (follow their directions)
5) Select the firmware image you want to use, and click next and agree to their terms.
6) Let it flash until it tells you to unplug and let the tablet restart

Should work (that's the normal flash routine).