most likely your old one is in /usr, so you need to pick a place (/opt/gtk-2.6 sounds good). I'd recommend giving it it's own directory like above, so if you want to remove it a simple "rm -fr /opt/gtk-2.6" and it's gone. you will also need to upgrade glib2, atk, and pango ( I'd install the all to /opt/gtk-2.6.
before you start, verify a file called xft.pc is in /usr/X11R6/lib/pkgconfig and do the following:
also go ahead and do the following:
the first one tells pkg-config (the program the gnome libs (and some others) use to set up include and linking flags) where to find what it wants to compile the new libs. the second one appends /opt/gtk-2.6/lib as a path the linker needs to search ( both when linking libraries at compile time and at runtime). note that we're not using ldconfig.
for the commands ( the order is glib, atk, pango, gtk):
./configure --help ( look over your options, though most of the gnome libs these days are straight forward)
make install ( or optionally make a rpm and install it)
when you configure pango, when configure is done it will print a summary of what backends are to be built. make sure xft is listed, or gtk will refuse to use it.
when you are done, you will need to set up a couple environmental variables to force the system to ignore the default gtk-2.0 libs and use your new ones. to do this, you can add the following to /etc/profile or another script that /etc/profile sources for the variables to take effect when you login:
you should be set with that, let me know if I forgot something.