Simply reply to the current schedule e-mail sent out by the instructor with your requested time slot for DEMO or DEBUG and indicate a second choice as well.
You may sign up for one DEBUG session at a time.
You may sign up for a DEMO session in addition to your DEBUG session request.
DEMO sessions are optional, but highly recommended since this is a chance to improve your grade compared to final results documented in your report (you can improve your project, show me what works well, what doesn't and why, which may improve your grade if you're able to better meet your originally contracted minimum, target, or goal objectives).