Having heard these words, king Bhagiratha directed his course towards the spot where lay those bodies of mighty Sagara’s sons, in order that, O most praiseworthy of men, the holy water might flood (the same). Having achieved the task of sustaining Ganga, Siva, saluted by men, went to Kailasa the most praiseworthy of mountains, accompanied by the celestials. And the protector of men (Bhagiratha) accompanied by Ganga reached the sea; and the sea, the abode of Varuna, was quickly filled.

In plain words: Bhagiratha led Ganga to where the remains of Sagara's sons lay, so her holy water could wash over them. Shiva went back to Mount Kailasa, and the king and Ganga reached the sea, which quickly filled up.