How many unit cells you need depends on the physics your FSS is based on. If your FSS is based on element resonance, you will only need 5 to 10. If it is based on array resonance, you could need dozens to 100's.
Hard to say how to fabricate without knowing what your FSS looks like, but most are fabricated using the same processes as standard printed circuit boards. When you test it, you will need two standard gain horn antennas and a network analyzer to measure reflection and transmission. Setup the two horns at some distance away from each other with nothing between them. Calibrate this to be 100% transmission. Now put your FSS a foot or so in front of the receiving horn and you will be measuring transmission through the FSS.