Guys,
I am very new to these mods, n900 and zero clue about linux. I have been trying to follow these threads like others and think I pretty much have my head around it and have been swapping fonts, it's great. I have seen a few guys struggling to grasp the install so simply trying to assist. I have used instructions by others and altered slightly for my own purpose.
I have created folders in my phone and attempted to be organised. So this adapted code works as per my folder structure and have highlighted in red. You can simply change the red to your own folder structure. Just make sure you change both codes portions in red. The first code will make a copy of the original and place in a folder (folder and path in red). The second takes the new files from the folder (in red ) and places in the correct location for it to work.

Firstly, download "rootsh" using your phone from Extras-devel and install.

How to activate Extras-devel
In the Application Manager:
1. Navigate to the application menu (tap the title bar)
2. Select ‘Application catalogs’
3. Select ‘New’
4. Enter a catalog name of ‘Maemo extras-devel’
5. Enter a web address of http://repository.maemo.org/extras-devel
6. Enter a distribution of ‘fremantle’
7. Enter components of ‘free non-free’
8. Select ‘Save’

I have created folders 'original' & 'options' under:
MyDocs/.documents/nokian900/n900mods/font/
Name the files you wish to use:
'nosnb.ttf'
'nosnr.ttf'
'nosns.ttf'
'nscnr.ttf'
and place in the 'font' folder.

Now open X-Terminal App on your phone.

(Make back up of original files using code below)
CODE:
sudo gainroot
cp /usr/share/fonts/nokia/* MyDocs/.documents/nokian900/n900mods/font/original/

(Now copy new files in it's place using code below)
CODE:
mv MyDocs/.documents/nokian900/n900mods/font/* /usr/share/fonts/nokia/

Reboot phone and reapply any widgets back to the home screen if they are missing. To apply a different font you must reapply the nokia original font first, (in original folder) and reboot phone, then apply your font files and reboot.

I hope I have not confused you further and assisted. Thank you to the others that gave us all the info, works like a charm.