Besides good lab practice, TAs and GTAs can influence how fulfilling the lab experience is for students by providing a positive learning environment so that students leave the lab feeling that they have learned/achieved something. While students often have a lab guide that prepares them for an experiment, an introduction is still required to set or reiterate expectations, and outline the purpose and aims of the experiment

To get students started:

Invite questions after the introduction to check/clarify understanding.
If an experiment has many parts, go through the first then get students together to discuss the other parts.
As the students are working on an experiment circulate around the room but don’t intrude unless you need to: let them get on with it.
Resist the temptation to butt in and solve the problems a student or a group might be having. If you see puzzled looks or people floundering, try asking questions: How’s it going? What are you doing/going to do? Why?
Try and see the problems from the students’ perspective.
If a lot of people are floundering, stop the lab and have a quick general discussion before proceeding.
Try to be aware of the whole room, so that you don’t inadvertently spend too much time with one student or group to the detriment of the others.

Check out the group work best practices in labs in Physics at Cornell University which is well transferable to other subject areas.

Facilitation of learning and responsibility of recording attendance