(1) To hold the crossbar to the turntable, tape works just fine.
(2) The RPM of the turntable should be discovered before class. It's easy to do just by sitting there and counting 10 rotations with a stopwatch. It should end up at a standard record player speed.
(3) The ropes need to be able to swing for a while so that any odd transient oscillations have time to die out. Tall students (apologies to the vertically challenged) should be picked to observe the position of the masses, and it should be made clear that they are to attempt to observe as close to the center of mass as possible. o.
(4) Everything that could interfere with the swing of the masses must be adjusted. This could include the stand, the meter stick, the power cord, and any miscellaneous objects nearby like desks.
(5) Tall students should be selected to observe the position of the masses, and it should be made clear that they are to attempt to make observations as close to the center of mass as possible.