The easist way to level a container that is to hold water is to first "rough it in" by eye.
DON'T DIG THE HOLE QUITE AS DEEP AS THE CONTAINER IS HIGH. The undisturbed ground underneath the container will help keep it from sinking. Use a board across the top to make sure the rim is above the surrounding soil level.
Now begin to fill the container with water - be sure that the rim stays above ground level.
A standing water surface is always level so just make sure that the water is evenly below the rim all the way around. This is very easy to see as the container nears full. (Works to see if a birdbath is level, too).
An occasional glance at the rim to water distance will also show if the container needs tweaking if it settles over time.