Review the rules for the college for which you work as a thesis adviser. Take notes on how the institution views your role. Read the rubrics for what the program expects of a master's degree candidate at an oral defense.
Ask questions of your superiors of any items you do not understand.
Write a letter to the committee members you are to lead. Ensure they understand the thesis defense judgment process -- what constitutes an excellent, passing or failing performance and the implications of such ratings for the presenter.
Compose a letter for your prospective student presenter. Let her know exactly how you and the committee are to judge her so she will know how to prepare for the defense.
Have the candidate open her presentation with a brief overview of her thesis. Ask her to explain her arguments and the process she used to come to her conclusions. Present opposing viewpoints to her conclusions. Take detailed notes on how she defends herself against such opposition.
Have your fellow committee members ask questions and make comments to her as well. Instruct them also to take notes.
Seek input from the audience members who are not committee members, if any. Invite them to ask questions of the student. Take brief notes on the questions they ask, and request the committee members do the same.
Request the candidate and the noncommittee members of the audience leave the room. Open deliberations by discussing how clearly the candidate presented her material, how well she fielded everyone's questions and defended her conclusions against the criticisms everyone presented. Grade her in each area, according to the specifications of the college or university for which you work. Instruct the rest of the committee to do the same.
Write a brief summary expressing the opinion of the committee based on the vote of the student's performance.
Invite the student back into the room. Tell her whether she passed or failed. If it's the decision of the committee she has failed, invite her to ask questions of you later. If your college allows her to revise her thesis for later acceptance, offer pointers on how to improve her report so she may improve her writing and/or presentation next time.