First of all you need to take measurements of your bathroom. In case the walls are not in good shape then in your measurements, determine space for finished walls.
Number of fixtures
Next you need to figure out how many fixtures you are going to install in your bathroom. You need to count sink or sinks, shower, tub or Jacuzzi and other fixtures that you need to replace or add. In case you are working on a half bath i.e. it only has a sink and toilet then you need to note down these two only. Determining the number of fixtures will tell you the number of places the plumbing is required.
Draw bathroom layout
Then you need to make a layout of your bathroom. You can do this either making it on a graph paper to scale or in a computer. By this, you will know where the plumbing is required and that there is enough space to walk. This step is also necessary to ensure that all places where plumbing is required are attended.
Incoming plumbing needs
After drawing the bathroom layout and knowing which area the fixtures are placed, you will figure out the incoming plumbing needs. You will know where both hot and cold water is supplied and where only cold is needed. You should install inlets in proper height and place. Remember toilet needs only cold inlet while sinks and tubs require both cold and hot inlets.
Calculate the drainage system
Next you need to carefully determine the drainage system. Every fixture requires a waste pipe which will be directed to the sewer connection or tank. You need to determine and install waste pipes of proper size and length.
Vent waste pipes
You should add a vent pipe with every waste pipe. The size of this pipe is determined after the number of waste pipes it is connected with. Remember that this process is difficult and complicated so it is advisable to use help from a professional.