After doing a fft of the data, simply set the coefficients of the transformed data to zero that are outside of your desired band.  Then do ifft of the modified coefficients.  This should give you a band pass filtering of your initial data.

Another method is to set a threshold minimum for the coefficients of your transformed data.  All coefficients with a magnitude below your threshold are set to zero, then take the ifft.  This will filter out random noise and concentrate the signal on its fundamental frequency components.  I hope these suggestions are helpful.