Although the product label will have general directions about how to use it, and in what concentration, it's a good idea to research directions for the specific type of plant you're trying to propagate before using rooting hormone.
In general, to propagate a plant with rooting hormone, first clip a piece from the parent plant with a clean knife or scissors — stem cuttings are likely to work best, but the hormone can also work on roots and leaves. Dip the cut surface of the clipping in the rooting hormone and then place it in potting medium, making sure to keep the medium damp, but not overwatered. Rooting hormone should never be used when propagating in water, since it will wash directly off the cutting.
Rooting can take a couple weeks to a couple months to work, but the roots will be more robust than if you used no auxin-boosting product. It's important to use the correct amount of rooting hormone, however, as too heavy a dosage can dehydrate or burn the plant's stems, change the plant's color or keep it from flowering.
Rooting hormone is available for purchase at most big box and smaller garden store retailers.