Reverse disassembly steps to put the cox box back together.

Ensure the O-ring on the back of the cox box is properly positioned before closing the back cover.

Use silicone grease on the O-ring if needed (can't hurt).

Make sure the O-ring is positioned properly - 99% of the time it doesn't matter, but if the boat flips and the cox box is under water you'll really wish you had been careful with the O-ring.