[header] How to plumb a bathroom [title] Determine the placement of the bathroom plumbing fixtures. [substeps] You will need to figure out the placement of your bathtub or shower, sink and toilet. This will determine where you will need to run the plumbing lines. 
You will have to cut holes in the floor so that you can run the pipes to the fixtures, so determining the exact placement is important. Locate and carefully mark all cut points and drill points measure again to make sure your markings are accurate.