The "MAIN" fabric is the one that doesn't get cut. It remains its full size and it is placed in the four corners. You cut 4.
The "SECONDARY" fabric is the one that gets cut in half. It fits between the previously placed main units along the edge. You cut 4.
The "CENTER" fabric is the one that gets cut into fourth. It is the one you see the least of and it is often the "pop" fabric. It fits in the center. You only cut 1.
Hope that helps.
Edit: Forgot to mention that you need to pay attention with the main fabric if it is directional. You need to swap out diagonal corners rather than turn the blocks.