Place the new power button in the slot so that the right end of the power button is under the plastic ridge on the right and the left end of the power button is on top of the plastic ridge on the left.

Make sure the power button is oriented the same way as the slot. The bottom left corner of the power button should stick out more than the top left corner, and the top right corner should stick out more than the bottom right corner. A simple way to check this is to held the power button in place and look at the side of the device (from the "outside" of the device).