Many websites explain how to report African violets.  Some sponsored by African violet societies are very detailed.  Others are not comprehensive enough.  The one that follows is neither too detailed nor too simplified.  We hope it answers your question.

http//www.extension.org/pages/35428/my-african-violet-saintpaulia-species-looks-like-an-umbrella-with-leaves-and-flowers-perched-high-abo

If the link to the website won't work for you, try copying it and pasting it in your browser's search box.  Otherwise follow these recommendations taken from a University of Rhode Island publication:

"Repot violets about once a year, or when the leafless portion of the stem is about 1 to 1 1/2 inches long. Gently remove the plant from the pot with a paring knife. Remove all lateral crowns, leaving only the center crown. Trim off one or two rows of leaves, leaving a wheel of leaves around the crown. With the paring knife, scrape the neck as you would a carrot until all the brown plant material is removed and the neck is firm and green. Break or cut off the bottom half of the root. Have a clean pot ready for the pruned violet. Gently firm in new soil around the plant and water well. The pot should be one-third the diameter of the plant; a 4-inch pot is usually adequate."

Go here to read the complete bulletin:

Caring for African Violets
http://www.uri.edu/ce/factsheets/sheets/africanviolet.html