You have to know about Bin fft.
If you have : FFT Size : 256 and Sample rate is 8000hz (the max frequency would contain on your data is 4000hz). then you have 8000/ 256 bin -> about 31.25 hz/ bin.
the High Pass Filter:
That means you want to get all data above f0 (= 2000 hz. Example) and remove all data below f0 (2000hz).
Bin at 2000hz = 2000/ 31.25 = 64 bins.
After you data put through FFT Block , you have frequency spectrum from -4000hz to +4000hz. and from 0 -> 128 -> 256
Then in frequency domain. you have to remove from 64 bins -> 128 -> 192 bins .
and you get bins from 0 -> 63 and 193 -> 256 ,
pass it into Ifft , So the output only contains from 2000hz to 4000hz .
I hope it helps you.