How to Create a Drone on a Harmonium
Many harmoniums have two or more drone stops, additional reeds that play even with no keys pressed. However, these almost always are the notes of the black keys, which is not what most Western players prefer.
On most harmoniums, you can create one or more drone notes simply by shifting the spring which returns the key to the up position to an adjacent key. The picture below demonstrates this on a Bhava harmonium with the C and lower G key spring moved to the left to create a drone in the key of C.