Here I'm flashing using the Global firmware. If you don't have skype pre-installed, its a good idea to flash it using a global version since it includes skype.
both of which can be obtained from http://tablets-dev.nokia.com/nokia_N900.php
3. flasher-3.5 which can be obtained from http://tablets-dev.nokia.com/maemo-dev-env-downloads.php
For detailed reference you can goto maemo wiki.
1. Install maemo-flasher.
2. Switch off the device, press 'u' on the phone's keyboard and plugin via USB. The phone will boot with a USB icon on the top right corner.
3. Run the following commands as required.
NB: The commands for flashing are (in order):
flasher-3.5.exe -F "path to eMMC image (vanilla)" -fwhich is for flashing eMMC, and then
flasher-3.5.exe -F "path to Combined image (fiasco)" -f -Rfor flashing the OS.
To flash the OS only, run the second command (fiasco). It is not recommended to only flash the eMMC (user content). So in that case, we need to flash the OS as well.
Download the video tutorial.