Steps:
1. Download a legal .ttf font from the internet
2. Make 4 copies of the font file and name them as below:
s60snr.ttf
s60ssb.ttf
s60tsb.ttf
S60ZDIGI.ttf
3. Place them in a folder called Fonts
4. Connect your phone and select Mass Storage USB connection mode
5. Browse to your Memory card (E) > Resource folder and place the the Font folder in there.
6. Reboot your phone to see the new fonts you've just installed.
Note:
Make sure that you connect your phone in Mass Storage mode to be able to see the resources folder.