boot Gentoo 2004.3 LiveCD, when prompt comes up, simply type cfdisk
the program will show you all ready formated windows drives/partisions, and below unformated 60/80, with the keypad move to the end, than with left, oops right key move it to the tab [New], create one 256MB for SWAP, and all others depend on what do u plan to install on your gentoo box. At the moment I have 3 partisions NTFS s 2003(15GB), 1 Debian GNU/Linux 2GB, and my Gentoo is installed in one big 45GB
u can make it and on separated partision how to handbook says, one for boot/ one for /var etc.
the last I think but not sure the ppl recommend it in case the part of the disk fails to save whatever u can(but most of the HD Drives in the world are Chinese (yellow) scrap, and if something goes bad with the disk (at least one cluster bad) throw it to recycle bin
anyway, the way to create new partisions for your Gentoo test box is called cfdisk (my recommendation - do not create more than one 250megs and one 5GB), after this 1st install u have to create 5-6 to create one really stable and cool Linux box in Gentoo way

If u cant give advice, dont send the helpless to the HOWTO's way or Google.com way.