Original:  After dessert (or after the fruit, if it is offered as a separate course), guests lightly dip their fingertips into the water, one hand at a time, and then wipe them on the napkin in their lap.
Revised:  After dessert (or after the fruit, if that forms the last course), guests lightly dip their fingertips into the water, one hand at a time, and then wipe them on the napkin in their lap.
Edit intent: clarity