You will need to seal the area underneath the pond before casting the resin. Otherwise, your resin will seep into the ‘soil’ and/or discolor. A dark plastic is what I would use.
Allow everything to cure before going to the next part.
2. The waterfall is going to be tougher. You will need to wait to pour your clear resin until it is quite thick. If it were me, I would set up something to be the ‘scaffolding’ for the waterfall, I would then pour resin onto the scaffold knowing that it’s going to flow and pool. This second part is much harder. I’m guessing there may be some ‘miniature’ forums that might be able to offer better advice in this situation.