Defining a notch filter

Hi, I'm new to Matlab. I'm trying to define a notch filter in matlab and implement it. The steps I've read for manually defining a notch filter so far are:

1- Taking the fft

2-Taking fftshift of the image

3- Defining the noise frequences in the fftshift image as notch centers

4-Implementing notch filter

I've done steps 1 and 2

I=imread('cameraman.tif');

F=fft2(I);

F = fftshift(F); % Center FFT

F = abs(F); % Get the magnitude

F = log(F+1); % Use log, for perceptual scaling, and +1 since log(0) is undefined

F = mat2gray(F); % Use mat2gray to scale the image between 0 and 1

imshow(F,[]);

Now I don't know how to get the frequencies to set as the notch centers. Kindly help!