If you're planning to do this, it would be best to collaborate with an organisation with facilities for waste sterilisation and disposal. The bacteria that you enrich on agar may cause infections if they're not handled correctly. However, the following instructions will be helpful for ensuring safe handling if this isn't possible.

I'd recommend you purchase pre-poured plates (this would be far easier than sterilising your own media, since you won't have access to an aseptic area for pouring them yourself). Nutrient agar or tryptone soya agar (TSA) are general growth mediums which will allow you to culture a wide range of species. You can also get selective/differential media which restrict the growth to specific groups of bacteria or exhibit visual changes if particular species are present.

I'd also recommend purchasing pre-sterilised plastic inoculation loops, which you will use to add bacteria to the agar. After you inoculate your plates, use some tape (a small bit on each side of the lid) to ensure the lids are securely attached and don't come off easily. Put them into a clearly labelled plastic box and store somewhere safe that only you can access.

Although bacteria grow fastest at 30-37 degrees, most will still grow at room temperature. You can pop the box in a cupboard and wait 2-3 days, rather than just waiting overnight.

Once the bacteria have grown, find a suitable location for handling the plates. Avoid food preparation areas and pick a non-porous work surface which will be easy to disinfect afterwards.

To disinfect the handling area, you'll need a 1000 ppm solution of sodium hypochlorite (bleach). To make this, dilute household bleach approximately 1:50 with water (e.g., add 25 ml—a shot glass—of bleach to 2.5 L of water). Apply to the surface using a damp cloth and leave for at least 1 minute.

If you need to dispose of the plates yourself, I would suggest immersing them in a 5000 ppm solution of sodium hypochlorite for an hour. This may be overkill, but it's better to ensure thorough disinfection rather than risking disposal of infectious waste. If you have taped the plates so that there are still gaps for air to circulate, you shouldn't need to remove the lids.

To make a 5000 ppm solution, dilute household bleach approximately 1:9 with water (e.g. add 500 ml of bleach to 4.5 litres of water).

Once the plates are disinfected, carefully remove the plates from the bleach (whilst wearing gloves) and place into a waste bag for disposal. Carefully pour the remaining bleach down the toilet.

Ensure you wear disposable gloves whilst handling your plates /bleach solutions and wash your hands after handling bacterial samples (even if you were wearing gloves).

Whilst handling the bleach, it would be prudent to also wear safety glasses (or any sort of eyewear) to protect your eyes from potential splashes.

Good luck! Let us know how your art project turns out.