How equalize on C after fft

问题:

first of all thanks for taking some time answering!.

My problem is the next: im trying to build an equalizer on C, using fft (fast fourier transformation), since now im able to apply the fft correctly on the vector (audio samples) normalize it and then apply the inverse fft. since here all ok , but when i try to modify the result of the fft(to increase the gain of one or various frequency bands) i get just noise (even if i just slightly modify it). How can i modify this bins (real+imaginary) components, that mi fft is giving:(0,real, imaginary, real, imaginary...) to equalize properly?

PD: IF someone needs to take a look im using this fft that aparently works fine: http://www-ee.uta.edu/eeweb/ip/Courses/DSP_new/Programs/fft.cpp

Thanks!