Part 1: Building A Vertical Dart Frog Vivarium HD - YouTube
1)Merge an aquarium or convert an aquarium? I'm assuming you mean convert and this video is a 4 part video I believe and it shows the basic process for doing so (Even though the video is for dart frogs).
2)That video sums up the false bottom. Get egg crate which I find at home depot in the lighting sections, they come in giant sheets and is relatively cheap.
3)You're gonna have to change the water almost every day. Not good to leave stagnant/contaminated water sitting in there. Frogs usually pee/poo in there.
4)Do a simple search of typical plants used and make sure they are not toxic to frogs. I personally recommend snake plants and bromeliads. There are tons more you can use so just search around.
5)Again the video shows how to make a background. I personally don't like the kitty clay method and usually only use great stuff. You don't need the pond and stone stuff you can get the normal yellow gap/crack filler great stuff and that's what I use.