Filter using DFT

I want to apply a filter using fft in small blocks. All I find on www is just to say fft on the entire data sequence. Is this the best way to do it.

 y(n:N+n-1)=real(ifft( fft( x(n:N+n-1) ).*spectrum_mirror ));

, where spectrum_mirror contains all frequencies including “mirror” frequencies

Before any complaint: I realize that it is wrong to ask someone about correcting code.