Choose the location of your basement bathroom carefully. If at all possible, plan to build it directly under another bathroom in your home so that it will be near the sewer lines. This can save you money, because you'll need to buy less material. It also prevents future problems with getting the correct elevation and rise for your piping.
Layout the room. Sketch the floor plan on paper first to work out logistics, then snap chalk lines to the floor to indicate where your walls will be built.
Build your walls using wood or metal studs and drywall. Use moisture-proof drywall to accommodate for the moisture associated with bathrooms and general basement dampness.
Purchase your plumbing fixtures. You'll need a sink, a toilet and a shower or bathtub if desired. Be sure to select fixtures that will fit within your available space.
Select an upflush toilet or a sewage ejector system if your bathroom will be below the level of your sewage pipes. This is the only way to get wastewater up and out of the basement.
Install your new fixtures and run all associated piping.
Hire a plumber to come to your home and make all final connections. In many areas, this is required by code. Even if it is not required in your area, it is often a wise choice so that you don't end up with a flooded basement. If you've already installed your fixtures and run the piping, it will be quick and inexpensive to hire a plumber for final connections.
Lay floor tile, paint your walls and apply any other desired finishes. Add towels and decorations to give your bathroom a cozy practical style.