The Microbit Soil Moisture Kit attaches to your Microbit. Here are the instructions for building them.
You will need the following:
Open the Microbit Soil Moisture package and take out the contents.
The board has been designed so that the Microbit can be bolted on using the 3 x M3 nuts and 3 x M3 screws. Place the screws through the P1, 3V and GND holes of the Moisture Sensor and Microbit, then use the M3 nuts to fasten together on the back of Microbit.
That's it, you're done!