Insert the switch contacts through the holes in the board.

Mind the switch's orientation! The actuator will go toward the right of the motherboard in this image.

Solder one switch contact to the board while pressing the switch into the board. Allow the switch to cool.

Ensure the switch is flush with the board, otherwise the mouse buttons will not click correctly. But also do not put excessive pressure on the plastic.

Solder the remaining contacts to the board, allowing the switch to cool each time.