The first and most important step is to turn off the water in your house!
After you're sure everything is turned off, you'll want to cut the the hot and cold water pipes that run to your bathroom. Also cut and remove all oft he existing water lines, the vent section leading to the sink, and the main stack 5 in below the vent tee. Take some old rags and stuff them into the openings to prevent leaking or smelly sewer gas.
Finish off the removal by cutting the main stack and everything connected to it. After everything is cut, emote the entire plumbing tree.
Drain all the remaining water from the pipes. Once you're sure no water is left, solder in two ball-valve water shutoffs. You can now turn on the water for the rest of the house without fear of flooding your bathroom.