You got it, Ant. Once the area inside the concrete beams is dug out, on goes the liner. You fill with water and work the liner in place. The liner goes over the top of the concrete beams. Then you use mortar to set stones or pavers or whatever over the liner and beam. The beam sort of acts as a foundation for edging of the pond.

BTW - Since starting work on the pond I've lost two kilos, from 82 to 80!