First,
Download the Nitdroid that you want, (For eg - nitdroid-0.0.9 and gingerbread)
IMPORTANT- You should enable the extra-Devel
Install the leafpad you can find it in app manager
then open,
x-terminal
root
apt-get update
apt-get install nitdroid-installer
If you don't find the gingerbread the click the site.
Nitdroid Gingerbread:
http://downloads.nitdroid.com/e-yes/gingerbread.tar.bz2
Copy both of these files to: /home/user/MyDocs

On a new X Terminal window, type the following commands:
Code:
root
cd /home/user/MyDocs
bzip2 -d gingerbread.tar.bz2
cd /home
mkdir /and
cd /
mount /home /and
cd /and
tar xvf /home/user/MyDocs/gingerbread.tar
dpkg -i /home/user/MyDocs/nitdroid-kernel-2.6.28-06_final1_armel.deb

Open the new terminal type:
leafpad /etc/multiboot.d/1 (then press tab on your touchscreen)

Now For the people how want to port two nitdroid on there n900:


root
cd /home/user/MyDocs
bzip2 -d NITDroid-0.0.9.tar.bz2
cd /home
mount /dev/mmcblk0p1 /and
cd /
mount /home /and
cd /and
tar xvf /home/user/MyDocs/NITDroid-0.0.9.tar

THATS IT ENJOY!!!

i dont mean to come across rude,
but this exactly isnt porting.

u also missed a huge chunk on how to configure multiboot properly.

there is also a huge thread on this topic already with a more thorough guide.
http://talk.maemo.org/showthread.php?t=66459