Thank you for helping to chair the sessions!
Here is a to-do checklist:
Consult with the organisers before the start of the first session if there are any special announcements.
Warn the speakers that you will raise a finger/hand if they are over time and apologise for rudeness.
Introduce each speaker (name, university, title of their talk).
Keep track of time. Generally research talks should be divided into two parts of 25 minutes + 25 minutes with a brief break in between. If the speaker goes over time, politely raise a hand.
Start clapping at end. Ask the audience for questions. Thank speaker again at the very end.
If there are no questions, it is the chair's responsibility to ask something! (Anything!)