The easiest way to root a tomato cutting is in water.
Simply place your cutting in a glass or container of room temperature water. No leaves should be submerged.
Place your cutting in a warm spot with indirect sunlight.
Monitor the water level, filling when necessary. Change the water every few days to keep it fresh, or change more often if it looks cloudy.
It usually takes between 10 and 14 days for your tomato cutting to develop roots. Leave the cutting in the glass for a little longer to let the roots lengthen to about an inch long before transplanting into the garden.