I’ve always used bits of info from several different sources when explaining how to slipstream SPs into install shares. The whole process is pretty much trivialized by nLite now, which can slipstream the SP and create the bootable ISO with just a few clicks, without even needing to really understand the concepts.
But I went ahead and wrote up a guide on how to do it manually. http://pctech.invisibill.net/slipstream.html should explain the whole thing pretty well. Let me know if you find any errors or if anything is unclear, so I can improve it.