The best way is to use the custom partition option. It may be best to let the installer make the partitions. Just click on the d: partition, delete it, then select it again and hit the auto button at the bottom. It should create the swap, the / and maybe a /home directory for you. They have a formula on how it does that but I have no clue what it is. I have never had it do the same way twice on different machines.
It has been quite a while since I used Mandrake. I switched to Gentoo a while back. If you are new, you may want to wait a bit before jumping into Gentoo. The install is not the easiest.
I hope that helps. If not you may wnat to start a thread in the newbie section. There are always more ways than one to do something.