[11] Snap in the tactile switches (clicky buttons). They’ll only go in one way and they’re symmetrical, so don’t worry about doing this incorrectly.
[12] Solder those buttons! You’ll notice the pads are bigger and will require more solder to cover. Also, there’s no need to cut these with the diagonal cutters; they are small enough.
[13] Snap the button caps onto the buttons.