Some key things to prepare is what precisely the professor wants. You already know 15+ min and wants you to speak publically to verbally convey ideas in a clear and coherent manner. But will your professor dock points for example if you go over 20 minutes? Have you talked to your professor about your stutter before? It might be a good opportunity to tell your professor about your stutter. Furthermore it may provide the option to let the professor know how able you are and want to do your presentation knowing stuttering is likely. Some other things to consider even though you’re a PWS, you aren’t alone. You aren’t alone as a PWS and you aren’t alone being nervous about giving presentations. Some people are already prepared to get a 0 or an F in your class, and you’re prepare even though you’re a PWS. That puts you in a good place regardless of if you realize it now. Furthermore practice and preparations helps tremendously. Your stutter is going to be the least of your worries if you don’t know the material and you didn’t prepare. Consider that every word you say is up for question and prepare accordingly. Also consider the same for the topics you present in the form of the “Five Ws” if possible.