First of all, you need to firmly pack the dirt at a levelled spot. You will use this place to put your birdbath in place. Take the birdbath and position it on the spot you selected.
Now take the large saucer and place it on top of the base, in a way the centre of the saucer meets the top of the base. The saucer will be used to hold the water.
It is really easy to clean, however, you will need something to hold it in place. For that purpose, you can fix a plant in a nice pot and place it in the small saucer. Then set both in the small circle of the larger saucer; it should be centred directly over the base.
The small saucer will keep the soil residues from the plant from flowing into the birdbath when you water the plant. It will also help keep the plant centred.
Fill the saucer of the birdbath slowly with water from the hose.
Shift the plant and saucers, if the way it fills indicates they are off-centre on the base.
When it is time to clean the birdbath just take off the plant/pot and scrub out the saucer.