I am not sure if I remember the plot correctly but these should be the basics:
A Women gets trained by a Mistress(and also dommed) to take care of a Salvegirl. The Slave is not allowed to speak. At some point the women has to take over from the Mistress and becomes the Mistress of the Slavegirl.
Source: reddit.com/r/eroticliterature/comments/pmoduo/looking_for_a_story_of_mistress_training_her