Thank you for your question about avocado. Below is the how to from the get go:

An avocado growing from seed should be cut back from the start. This is the best way to achieve a many-branched, well-shaped tree.  When your new plant is 6 or 7 inches high, cut the stem at a point midway between its top and bottom, using scissors. The remaining stem should be no shorter than 3 inches high. The plant will take about a week or two to start growing again but it will then grow into a nice, small bush.

A mature plant should also be pruned regularly. Cutting back keeps the plant in an attractive shape. Cut back both the growing tip and any side branches that have become too long. All branch cuts should be made close to the trunk or branch. Do not be afraid of cutting back too much; the plant will eventually recover.

Hope this is helpful. If you have more questions let us know.