With calculator face down, replace the button membrane, and then the circuit board.

Then press the back of the case on and snap it together.

There are 4 little posts on the back of the case, these align through the circuit board and button membrane, so make sure those are lined up.

It might be tempting to assemble everything on the back of the case (with posts aligned) and then try to press it into the front of the case. But it will be hard to keep the two pieces from falling out while also trying to keep all the keys from falling out!

Snap the faceplate back on.

Install the batteries.