Only the foolhardy or the brave attempt their own plumbing. I firmly fit into the foolhardy category. However, having successfully completed my own plumbing project, I feel imminently qualified to tell everyone else a step-by-step process for switching rusting, cast iron pipes for PVC pipes.
The first step is to ponder the problem. Don't look at the pipes. Don't read any DIY material. Simply ponder it. The second step is to run cold water. Then run the hot water. If you're lucky, the pipe fairies have flown through and declogged it for you. Step three involves testing every flashlight you own to find a single working one. Once an adequate -- and working one -- has been found, you analyze the pipes in question.
I'm guessing that my house had been modified or added on to four or five times before my family acquired it. Originally, it was what you called a "shotgun" house. (I still refer to it as a "shotgun" house, but for different reasons!) Consequently, the pipes under the house look like something out of a Dr. Seuss book. To analyze my pipes, I had to crawl through a veritable maze of tubes, pipes, duct work, dirt and spider's homes to find the offending pipe.
For the fourth step, get out pen/pencil and paper and sketch out the problem. Once sketched, turn the paper in every conceivable direction. It looks smart and makes your spouse think you're being productive. Step five consists of visiting your brother-in-law for a saw, a neighbor for pipe prep and cement, the hardware store for supplies and the library for a book on how to replace rusted pipes. For step six, line up all tools and equipment for a visual inspection of all items necessary. Be sure the saw is charged and the tools are in order from smallest to largest and have a nice shine. Step seven will have you meditating and mentally prepping yourself for the task ahead.
Step eight is the most crucial. It involves a brutally honest spouse telling you to quit piddling and get on it! That's when you get your butt under the house for several hours and fix the pipe, only to discover that there was a massive clog in the wall pipes behind the sink which will only take 45 minutes at most to clear. And, voila! You've fixed your pipes.