Make sure you measure the length of the pendulum from the centre (of mass) of the plumbob.
Make sure the top edge is well defined- clamp the string between two blocks so it is clear where the string can move and where it cant.
Make sure the top support is fixed- eg a clamp stand can easily flex and so the top support moves.
Make sure you use small amplitudes ( less than 10 degrees from the vertical)
Make sure you time enough swings such that the total time is sufficeint to get the uncertainty in the timing of a single swing down. (It is the total time that is important not the number of swings)
Release the pendulum and start timing counting while it is swinging. Start the clock and say zero (not 1).
Have a pre-prepared results table so that you can record your results accurately without rushing.
Do at least 3 sets of readings to identify any anomalies.
Step-by-step explanation: