Plumbing might seem to be relatively easy, however little details can mean the difference between a system that works and a system that works but will have problems in the future. For example, drain waste lines or DWV (Drain-Waste-Vent) piping is often installed with a pitch that is too steep. It might seem like "the steeper the pitch, the better it will drain". However, this is not true. Solids in a drainpipe must be allowed to "float" in a column of water as they move down the pipe. This will allow the waste material to "clear" the drainpipe and not be allowed to settle in the pipe and possibly cause the pipe to eventually clog. The slope recommended is 1/4 inches of drop for every linear foot of drainpipe. So, a 10-foot length of drainpipe should drop (10 x 1/4) 2.5 inches for proper drainage. Any more drop than this can allow solids to stick to the pipe instead of being "floated" out. Any less of a slope might not allow proper flow of the water and solids.
Also, you can't just route drain lines wherever you want like you can water supply lines. Water supply lines have water "Pressure" in them allowing their installation around obstacles. Routing drain lines around obstacles inproperly can lead to clogged lines in the future. To avoid plumbing issues, ALWAYS HAVE PLUMBING DONE BY A QUALIFIED CONTRACRTOR.