Slice the avocado along the length of the fruit. Use a firm even pressure, but be careful not to cut into the seed.
Gently pry the two halves of the avocado open revealing the pit.
Remove the pit from the flesh and wash off any remaining bits of flesh.
Fill a glass or tumbler with fresh water to one inch of the rim.
Hold the pit with the rounded end down. This is the end that will sprout a large tap root in a few weeks.
Locate the center of the pit and insert 4 toothpicks to a depth of 1/4-inch at equal intervals around the circumference of the pit. The easiest way to do this is to insert two toothpicks directly opposite each other and then add the other toothpicks in between.
Set the avocado seed with the attached toothpicks into the glass. The bottom of the pit should be covered half way with water. Add more water, if necessary. The toothpicks will provide support for the pit until it roots.
Place the pot on a sunny window sill.
Change the water every 3 to 4 days and be sure the bottom of the pit always rests in water.
Watch for a large tap root to sprout from the bottom of the pit. This may take 3 weeks or more. The pit may crack or split in half at this point. Soon afterward a shoot will appear from the top of the pit and unfold into a leaf.
Allow the plant to grow in the glass until it has established a good root system and the top has several leaves.
Plant the seedling in all purpose potting soil so that the top half of the pit is above the soil level.
Water the plant regularly whenever the soil dries out. Avoid over-watering.
Pinch out center leaves to encourage bushy growth if the seedling becomes tall and spindly.