When the avocado fruit has turned black and is ready to eat you can remove the seed. Take a knife and half the fruit from stem end to bottom twisting gently to separate the halves from each other and the seed. Use a sturdy smooth blade kitchen knife to whack into the side of the seed and give it a turn to release it from the meat of the second half.
Wash it well in cool water. Don’t store these too long or they may not sprout. Usually, you just stab 2 or 3 toothpicks into the sides of the seed to suspend it over a glass of water. Make sure that the water covers the bottom half of the seed at all times.
On a side note, I learned something very interesting the other day on Alton Brown’s Good Eats program on the Food Network. The avocado fruit will not ripen until it is removed from the tree. This allows farmers an almost unending harvest season as long as the weather holds up!