Cut a 6-inch tip from the stem of a parent plant, using clean, sharp scissors. Snip off any bloom buds on the stem and remove leaves from the lower half of the stem. Take the cutting from a healthy, mature basil plant in spring or early summer, before young green stems turn woody.
Put the stem in a clear glass or jar of water. Don't allow any of the leaves to touch the water.
Set the cutting on a warm, brightly lit windowsill out of direct sun. Your basil will root in seven to 10 days.
Change the water every day while the cutting is rooting.
Fill a 4-inch pot with sterile potting soil when your basil cutting's roots are about an inch long. Plant the cutting in the pot and set the pot in a shallow container of warm water until the surface of the soil feels barely moist. Remove it from the water and allow it to drain freely.
Keep your basil plant in a warm, brightly lit spot. A sunny windowsill is perfect. Water just enough to evenly moisten the soil when the surface begins to dry out. Don't let the plant get wet feet.