Go to "support.dell.com" and enter the service tag number - OR enter the model number and follow the prompts. Then you can download the Service Manual and proceed from there.
I would suggest that you take some digital pictures of the existing setup before you start disconnecting stuff. And keep taking pics as you disconnect. This is what I did with my Dell Precision WS 490 when I added a drive, and did some modifications. Also, mine was purchased used, and the insides were covered in oil. I took out the fans and cleaned them with solvent, and did the same to the interior.