Simply put some sand in the bottom of the jar (colored play sand works great for this) and add the seashells.

只要将沙子铺到瓶底，(如果是彩沙就更漂亮了)然后再放入贝壳。

youdao

They had an old birds cage, and it had a jar at the bottom.

他们有一个旧的鸟笼子，笼子底部有一个瓶子。

youdao

To my amazement, there, as if it had never been removed, stood the old pickle jar, the bottom already covered with coins.

令我感到惊讶的是，那儿放着那个旧泡菜坛，坛底已经铺满。

youdao

The butterfly would flutter for a few moments, sink to the bottom of the jar, and slowly expire.

NPR: Excerpt: 'At Large and At Small'

There ought to be at least a nickel's width of clearance between the whirring blades and the bottom or sides of the blender jar.

WSJ: How to Ace a Google Interview

At the bottom of the chest, in an olive jar, he placed a detailed autobiography, printed so small a reader will need a magnifying glass.

NPR: 'I Love Mysteries,' Says Man Claiming Hidden Gold