Clamp & stand
Cotton or string
Depending on the height of your stand set the range of lengths of string you will test - 5 to 50 cm is a suggested value.
Clamp the string at the desired length and raise the pendulum, keeping the string taut.
Release the pendulum and start the stopclock. Time how long it takes for 10 swings to complete, remembering that one swing is a complete motion from left to right and back to the left again. Timing 10 swings (or more if you wish) will help reduce the error per swing.
An alternate method to improve the timing would be to set the pendulum up with a light gate and datalogger, so that the pendulum bob breaks the beam of the light gate. This would only time one swing.
Take three or more sets of results for each length to enable errors to be spotted and eliminated when you calculate the mean value for one swing at each length.
This experiment is generally safe, with the only real hazard being knocking over the clamp stand. This can be avoided by placing it in the middle of the bench, or alternatively using a g-clamp to secure it. It is advised that large mass bobs are not used as this will also affect the stability of the experiment.
Finally plot your results on a graph. If you have time you may wish to play a second graph of bob length against period squared - this second graph will end up as a directly proportional line if you have made your measurements carefully enough.