How do I implement IFFT to filter low frequencies from a group,after Freq detection by FFT?
Frequency domain processing (i.e FFT, manipulating signal, IFFT) is actually pretty complicated and requires careful choice and a thorough understanding of things like framing, step size, windowing, overlap, wrap-around, circular vs. linear etc. Simple examples for these type of algorithms are overlap-add or overlap-save for linear time invariant filtering.

If the only thing you want to do is getting rid of the low frequencies in your signal, filtering with a high pass filter is much easier and quicker.