To drag a bucket into another, click and hold the bucket and drag it into another main bucket. You will see an orange outline when you are over the bucket you want to drag into, and you can then release it to complete the move.

You will get a warning message as this action can not be undone

The dragged bucket will then become a sub bucket of the dragged into bucket.

If the bucket had sub buckets they would also be sub buckets of the bucket you dragged it into.