If you are rewriting a paper, based on your professor's comments, do that first, and bring both versions with you. This way, the Writing Assistant can help you to determine if you have responded to all the comments. Leave yourself plenty of time, especially if you are having problems with the assignment. Plan ahead and come in at least two days before the due date. Be realistic in your expectations of what you and a Writing Assistant can do.
Assistants can point out problems, discuss alternatives, and make suggestions.
But you must put in effort during the conference and in follow-up revision, if this work is to produce improvement in your writing. Keep in mind that an Assistant's help, while based on a competency in writing, is neither the final word nor guaranteed for accuracy. The ultimate responsibility to your work lies with you.