Please help! How to do the measurement for FSS? (I want to measure S11 and S21)

How to do the measurement for FSS? (I want to measure S11 and S21)

I have a sample of FSS and I want to measure reflection and transmission in Ka band.
I heard that I can use two Ka band horn antennas with VNA. However, I don't know much about the measurement ,for example, the distance between two horn antennas and the distance between horn antenna and sample.

Thank you very much for your help