To clone a houseplant, snip off approximately 2-4 inches of a stem. It should be cut at an angle to allow maximum root growth. It's optional, but you could dip the cut end into root growth stimulator. Then, plant this stem into a pot of soil. Water as usual and the new plant will be a clone of the old.
Not all house plants can be cloned, but most of them. Some plants can be cloned by taking stem or even leaf cuttings, some plants can be divided. Most cutting requires bottom heat or rooting hormones to be rooted.