A simple pendulum is an arrangement, wherein an inelastic thin string of ideally no mass with a heavy point bob at one end and tied to a fixed horizontal rod, hanging freely and is free to make movement about mean point.
The precautions while measuring period of oscillations:
Record time for arround 20 oscillations. And record the time for a repeated number of 20 oscillations and find the average period of oscillations.
The length of the pendulum should take into account hook, sring length, and radius of the bob.
The sring should not be streched too much while measuring its length.
The amplitudes should be very small, or the angle of the amplitude should be very small.
Importantly is the angle at which the pendulum is drawn out to. The smaller the angle, the closer to harmonic motion it will behave because the restoring force is acting tangent to the arc the pendulum moves through instead of being outside of it. therefore, keep the angle at which you draw out the pendulum less that about 12 degrees.
also, the period of the pendulum is based on the length of it so make sure that the string is one that does not stretch. At the bottom of the swing, the string can stretch affecting it's length and therefore affecting its period. the tension in the string can be calculated T = mg + Fc where Fc is centripetal force. The stretch needs to be eliminated by using a high test fishing line or other, non elastic material.
A third factor of course friction. Use materials that do not have much friction like the fishing line.
finally the shape of the bob is important. the pendulum's mass is measured to the center of mass of the bob. A small, spherical bob is much preferred over a long one. Small lead fishing weights that are spherical are ideal. they also eliminate or minimize a lot of error due to air resistance.