Sure, here's a video:
https://imgur.com/a/GKJZpqx
I feel like I'm doing what I've done with the rest of the spots where I could use items; hit Z in order to check the spot, make sure the item I want to use is in my inventory, then press Enter.
In this case, I'm pressing Z to make sure I'm in the right spot with the plant, and then I'm pressing Enter to use my item, with nothing happening. Then I switch to the candle, and then I'm pressing Z again to make sure I'm in the right spot, and then I press Enter to use the item.
I also found that I can press enter outside of the caves in order to get dialog about using the candle/water, but using items inside the cave doesn't seem to work for me outright.
Of course, there is an 11/10 chance I'm in the wrong spot and that I'm doing it wrong, in which case, terribly sorry about this whole thing, but I can't figure out where else I might need to go in order to use these items.