You can.
I like to use cfdisk actually, but fdisk is fine.
Do like fdisk /dev/hda or cfdisk /dev/hdc or whatever. Remember there is no "c" drive in *nix, and the hardware resources corrispond to special /dev/ files. Hd* files represent IDE devices (cdroms, zip drives, harddrives, whatever).
hda=primary master
hdb=pimary slave
hdc=secondary master
hdd=secondary slave.
Then each partition for each harddrive is represented by a file with the hd* with the partition number added on the end. Like /dev/hda1 would be the first partition on the primary master harddrive and /dev/hdc3 would be the 3rd partition on the secondary master, etc etc.
I make one big / (root) partition for all my files, a /home partition so that I can format and reinstall the OS without messing up any user settings or files and a swap partition. You won't need a swap partition any larger then 512 (you can have it bigger if you want, though) megs or so, irregardless of how much real ram your running.
I like to write down the drive/partition names and what I want to use them for so that I don't get mixed up and forget which was which.
Once you get everything partitioned out, reboot if you want to make the changes perminate (not neccisary, but its a nice thing to do).
Once you boot up with the slackware install disk you would just type it "setup" if I remember correctly. It should have a couple pages of explaination you can look thru at boot up.
It will have a bunch of steps to go thru, you can skip some of them, but generally just go thru one by one
Once you get to the "target" stuff, format the partitions partitions as ext3 partitions, except for the swap of course. You can skip the sector scans or whatever if you don't want to wait and trust your harddrive to be healthy.
Once it becomes time to select what packages to install, just tell it to install everything and you will be fine. (uses up to 4 gigs or so, maybe more, I forget how exactly big it gets with "everything")