Start with these from the Tutorials:

• A Guide to Installing Modules
• A guide to installing modules for Win32

To find ppm's, you can increase your repository list w/PPM::Repositories .. also, google will often find a .ppm file for you -- try google(GD::Graph ppm), and then you can open ppm3 from a dos prompt and do install http://somesite.there.com/blah/foo.tar.gz

In general, to install manually on win32 (though the second tutorial above explains why it's probablly better to use ppm), you just extract the .tar.gz into a temp dir, and change into the extract directory (GD-Graph-*), and do:

perl Makefile.PL
nmake
nmake install
[download]

If you don't have a nmake.exe, see the link ww posted -- it has info on obtaining make for win32.