You're going to need:
- Suction cup shower caddy with drainage holes (I got mine at Target)
- Damp sphagnum moss (or soil)
- Fertilizer (I'm using a slow-release conventional one)
- Cuttings or potted plant
When choosing your plant, you want an indoor plant that enjoys high humidity such as Christmas cactus, rhipsalis and other epiphytic plants, and you want to avoid plants that grow fast and large - try to stick with the slower growing ones that stay small for a while. I'd also recommend trailing plants so the caddy is eventually covered and disappears for a beautiful effect. However, you may alternatively decide to go with a rhizomatous plant like rabbit's foot fern.
I'm using sphagnum moss because I have some from potting up carnivorous plants that naturally grow in wetlands alongside sphagnum, and I'm further using it because it's lightweight and doesn't release tons of debris when watered. I have considered using natural wool or synthetic yarn as an alternative to sphagnum but haven't tested the idea.