how to apply Cross-correlation, Auto-Correlation and Windowing

Dear Experts.

I am trying to to do auto-correlation and crosscorrelation in frequency domain.

I have to apply windowing also to avoid the spectral hole.

I need stepwise valuable suggestion for this approach. because I got puzzeled right now.

1. How to apply cross-correlation in frequency domain.

2. I want to apply hanning window. How to apply hanning window in frequecy domain.

which approach is best like hanning window in time domain or frequency domain.

what is the meaning of order of hanning window?

3. Do I need to apply IFFT in output of cross-correlation and auto-correlation function?

4. I have to divide the cross-correlaton with auto-correlation function.

I thick it should be done in frequency domain. Am I right?

I need help to understand the concept.

thank you.