After you've dry-fitted the three main boards of the cutting board, you'll need to cut the tongues on the ends of these three boards.
With the dado blade's height set to the same final height (that gave you a tight fitting joint) in the previous step, adjust the table saw's fence to 3/4" from the edge of the blade. Clamp a short piece of 1x stock to the leading edge of the fence, as shown in the image above. This clamped piece should be clear of the saw blade, and will serve as a spacer.
Using your miter gauge (set square to the blade), place board #1 on the table against the miter gauge and also against the clamped board on the fence. Then, while holding the piece tightly against the miter gauge, slide the combination forward and through the blade. Once you've cleared the blade, lift the board, slide the gauge back and place the board onto the table to cut the opposite side of the tongue. Repeat with each end of each of the three center boards.
Dry fit the entire board together and make certain that the tongues fit snugly but not too tightly.