well, first of all you have to make room for debian (say 2GB for binaries + 500M for /home, which you may share among mdk and deb)
actually resizing a ext2 fs is not so easy, so if you can, steal it from a fat.
Take a written note of which partition will host which os/mountpoint
Then install debian sharing the same swap as mdk.
You may want to mount mdk / on /mnt/mdk, to take inspiration from its /etc folder (be careful: mandrake is not too clean).
When it comes to choosing bootloader, install it in debians own partition.
you may whant a bootdisk, too.
then reboot in mdk (so you have an already configured system to work in)
and edit your bootloader config file.
i'd reccomand grub as a bootloader for such a system: it's much more flexible.
Add your debian entry to menu.list with either kernel path or chainloader to deb partition (if you installed lilo in there)
see (and maybe print) man grub for details
If you fail there's no problem, grub will always be able to boot your system even with corrupted config file.
enjoy your deb
PS: woody is getting old, if u have a good internet connection cosider upgrading to sarge (testing), despite its name it's fairly stable.