How do you partition a sd card that already has data on it?

Hi I am considering double partitioning my sdcard. I am looking for a starting place and have so far not found any information on this subject. If you know of a good thread on partitioning would you post it. I am a noob, have droid rooted cm 5.0.6.2 Dark Droid SE 24K, overclocked 1000MHz. I could use Rom Manager (at leaste the ability is listed, it said it would wipe out all date so I am staying away from it for now until I know how to back up my sd card. So any one who has done so please help. Thanks in no hurry