Outside: Just need to put seed upright in the soil and water it frequently.
Plant it in a non-lawn area and away from sidewalks and, if you can, plant it in a spot protected from wind and frost. Remember, full sun is best. Dig a hole as deep as the current root ball and just as wide as the width plus a little extra so you can get your hands into the hole to plant it. The avocado is a shallow-rooted tree with most of its feeder roots in the top 6 inches of soil, so give it good aeration. Its root system is very sensitive, and great care should be taken not to disturb it when transplanting. As with most plants, you don’t want the tree to get too dry. The rule of thumb for mature trees is about 20 gallons of water a day during the irrigation season. Seedlings will require quite a bit less water, of course. Check the soil before watering each time to make sure it has dried somewhat. If the soil from around the roots can hold the impression of a hand when squeezed, it has enough water. If the tree is root-bound, however, loosen up the soil around the edges and clip the roots that are going in circles. As fertilizer, ordinary home fertilizer for houseplants will work fine.
Starting as Houseplant:
Use three toothpicks to suspend it broad end down over a water-filled glass to cover about an inch of the seed.
Put the glass in a warm place out of direct sunlight and replenish water as needed. You should see the roots and stem start to sprout in about two to six weeks. (If you’ve followed this process so far and have not seen roots or a stem sprout in more than six to eight weeks, try another seed.)
When the stem is about 7 inches long, you can cut it back to about 4 inches. When the roots are thick and the stem has leaves again, plant it in a rich humus soil in a 10½-inch-diameter pot, leaving the seed half exposed. Water it frequently, with an occasional deep soak.
The soil should be moist but not saturated. And don’t forget: the more sunlight, the better. If the plant turns yellow, you may be over-watering; let it dry out for a few days. If the leaves turn brown and fry at the tips, too much salt has accumulated in the soil.
Let water run freely into the pot, and drain it for several minutes. When the stem is 12 inches high, cut it back to 6 inches to encourage the growth of new shoots.