Product review (5 stars): Easily Installed
As long as the humidifier is positioned such that you can position yourself so that you can look down on the top of it, the Water Sensor Assembly is very easy to install. Basically, with the unit unplugged, remove the cover (one screw), disconnect the sensor wire, lift a retaining clip, then lift out the sensor, and take out two gaskets from under the sensor. Then reverse the process to install the new one. All of these parts (except the cover) come in the kit, though you might choose not to replace them all, e.g. the screw or the retaining clip. (And yes, part -002 is the correct one; it replaces part -001 which your manual probably tells you to get.)

If the humidifier is so close to a ceiling that you can't get in a position to look down on the top of it, things could get a little more complicated: you might have to remove the humidifier to change the assembly as, in particular, it would be difficult to position the gaskets by feel.

Also, before replacing the sensor, you might want to file the terminals of the old one to remove sediment, corrosion, etc., so that they are shiny metal; that may be all that's needed to get the old sensor and the humidifier operating again.