Remove the cover of the cushion to find the battery circuit. You need not remove the entire cover. You can easily locate the circuit inside the cushion.
Hold the battery and plug it into the circuit by matching the size of the point. Press it and you will see the battery fitting in smoothly. Once done, switch on the button to check.
In case if the cushion does not show LED light, you may have to press the battery tightly into the circuit. The on/off button is inside the cushion & You can replace the batteries as and when required.