Lay the motherboard flat on the table, as shown.

On the chassis, make sure that the green antenna wire is in its slot, and that the touchscreen ribbon cable is properly positioned.

Lay the opened chassis on a table near the motherboard and gently separate the upper and lower motherboard ribbon connectors.

Still holding them separated, slide the motherboard into that separation, so that only one ribbon connector is above the motherboard, and the 2 plastic pins are resting in their holes.