Advice for normal map baking

Hello.
I have a box shaped model and I need to bake normal map from the high poly mesh. I have go through some tutorials and I confused about having low poly model with flat shading and smooth shading when baking normal maps. I’m going to use those objects on cryengine, so I’m using blender 2.72 cycles baking with x+ y- z+.
Can anyone give me step by step instruction how to prepare my model for baking? Thanks.