Certified GKH Gardening Expert
This can be tricky, as these plants are VERY sensitive to change. The way I do it is very intensive, and requires your attention for multiple days. Cut the leaf or flower right where it joins the stem, and slowly cut back the stem as it dies. This will allow the plant to take back valuable nutrients from the stem as it slowly dies.
Here is an article for more information on the proper care of these plants: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm