It will be helpful if both.
I know the formula of the Power, but what I am after is to where in the circuit and how will I connect my WATTMETER?
Because I tried connecting and simulating it on the output part and it reads ---.--uW.
You would interpose the wattmeter between the amplifier output and a purely resistive dummy load which replaces the speaker - it might be 8Ω. You would have to inject a suitable signal at the amplifier input. The input signal level (say using a 1kHz pure sine wave source) would be increased until the power output was as expected, but not at a level where unacceptable distortion of the amplified version of the input signal is observed at the load. So you would need to assess the output signal quality (harmonic distortion) to make this a genuine test of output power performance.