Hi Nos. I use to add layers of soil, then substrate (either coco husk or jungle mix) then some sort of moss (sphagnum or pillow moss), all this over a layer of mesh which should cover your hydro balls (to prevent being soiled by the substrate).

Lately though, I have changed my ways, as I find the plants get to wet the way i was doing it. I now mix coco husk with soil with moss in a 1:1:1 ratio, instead of layering each on top of another. It creates a humid but less compact substrate and my plants are much more happy.

As for the backdrop, you can either use bark, or jungle mix glued by foam and or silicone II. I've seen clay, but I wouldn't advise it if it's your first viv. There are many threads on this forum which will detail each step of a vivarium build.

Do you know which frogs you are getting?

Good luck!