Copy the font files to a directory that is visible to the x-server's
Font Path.  You may check the font path of the running server by
typing the command:

xset q

On my FreeBSD machine I chose /usr/X11R6/lib/X11/fonts/misc/ to hold
these new fonts.  Next, you need to update the index file 'fonts.dir'
by running 'mkfontdir'. Here's the command I used for my path:

mkfontdir /usr/X11R6/lib/X11/fonts/misc/

If you don't want to restart your X server, you can update your
current X session using:

xset fp rehash

Otherwise restarting X will also load the new fonts.  That's it, you're done.

=== Addendum ===

At least for Ubuntu Linux you need an extra step.
You must also add the font path using "xset fp+".
Here's a shell script that does the job:
    #!/bin/sh
    # rebuild font.dir file
    mkfontdir ~/.fonts
    # add .fonts to font path
    xset fp+ ~/.fonts
    # tell X to reread fonts
    xset fp rehash

Thanks Noah.