I'll expand on what Obscure said.
There are two font management systems you have to worry about. Fontconfig has one, and X has one. You deal with the two of them separately.
ADDING FONTS TO FONTCONFIG
Fontconfig supplies fonts to your gtk2 and qt programs. First, put your new Truetype fonts into a directory. Then look for a file called /etc/fonts/fonts.conf. Read it. Learn where it searches for fonts, and how to add a font search path to it, but don't edit it. Instead, make your changes to /etc/fonts/local.conf. Then run "fc-cache -f -v" as root. The new fonts should be accessible now.
ADDING FONTS TO X
To add fonts to X, you need ttmkfdir
. Put all your new Truetype fonts into a directory. Then go to that directory and type:
ttmkfdir > fonts.scale
ttmkfdir > fonts.dir
If you look in /etc/XF86Config, it will show you which directories X looks in for fonts. It will also become obvious how to add a "fontpath" to it. Add your new font directory, which contains your fonts and your fonts.scale and your fonts.dir, to your XF86Config's fontpaths. Then restart X.