He told me like "Do FFT and then IFFT to make sure FFT is working well".
If you work a bit at this, you will end up understanding the material much better than your instructor.
You absolutely must have a good low pass filter in the electrode/amplifier setup that prevents any signals with frequency higher than 1/2 your sample frequency from entering the Arduino. Otherwise the results will be uninterpretable.
For example, if your Arduino sample frequency is 100 Hz, under no circumstances can you allow a signal with frequency greater than 50 Hz get to the input.
Look up "sampling theorem" and "Nyquist limit" for more information.