That’s a good video, I’ll keep that in mind for when I do mine! A waterfall is a structure, and needs a good base, with strong framework. Start at the bottom and work your way up. For the taller vertical areas, I’ve seen several things you can do. Either a few tall rocks, lots of rocks all stacked, ( bigger ones on bottom leading up to smaller ones) or I saw somewhere where a person mixed sand, small rock into a thing of waterfall foam, and used a putty knife or trowl to apply it. You could stick small pebbles in it while it’s still wet, creating a look of a natural earth wall.