How to implement grounding into this electrical circuit?
In most electronics and electrical circuits, "Ground" or "GND" is just the point in the circuit that we choose to call "zero volts", and use as a reference when measuring voltages elsewhere in the circuit - it is where we put the black meter lead.

Very often, "GND" is the most negative terminal of the power supply, but if the designer chooses, may be the positive terminal, or perhaps the mid-point of the power supply.

There is no "GND" part, and "Ground" has no special or magical properties.

In AC power distribution, and in some radio antenna systems, "Ground" does indicate a connection to the earth, but that clearly doesn't apply in portable battery-operated circuits.