Low pass filter design in frequency domain

Dear all

can someone please tell me how to implement a low pass filter in frequency domain?
i hav done the following steps:
1.FFt of the image
2.Multiplying by a fn like (1+cos*pi*x/window size)/2
3.Ifft 2 spatial domain

If i want to designa  low pass filter of window size 12 by 12, 16 by 16..etc
how do i do this???please help me.

thankyou very much

Vijay