I'm a high school student trying to model a note played on my instrument using a Fourier Series. So far, this is my code in MATLAB (I haven't included the code that keeps the FFT plot within the Nyquist limit, but when I plot it it shows up correctly).

https://pastebin.com/YPP0dR9T

When I return m, as expected, it returns the magnitudes of all the complex numbers in my vector X. However, now that I have these FFT values, I have no idea what to do with them! Ie, how do I take these values and represent them using a complex exponential or even real Fourier Series? (I would appreciate knowing how to do it with a real Fourier Series as it means there's one less thing to do, but I guess I could convert from complex exponential to real if absolutely necessary.)

Thank you so much for your help!