I can't find the definitive thread but basically what you do is the following
- With the Kobo attached to your PC create a directory in the ROOT directory called fonts LOWER CASE
- Copy the 4 files that correspond to the font to that directory. The four files need to end with -Bold, -BoldItalic, -Italic, or alternatively, b, i, z.
- For instance Arial.ttf, Arial-Bold.ttf, Arial-BoldItalic.ttf, and Arial-Italic.ttf
Then eject the Kobo and you shoujd be done.
[EDIT]
Updated to correct the directory name