Display: A ten gallon aquarium seems to be the standard of measure that most people are familiar with. Ten to twenty adult millipedes can live in a cage this size. A cage this size is also suitable for a single millipede. Baby millipedes can be kept in large numbers in fairly small cages.
Various plastic shoe box or storage containers world well with millipedes. As always, I prefer transparent ones so that I can monitor the contents without necessarily having to open it.
Substrate is the term used to describe what you line the bottom of the habitat with. I prefer coconut fiber as it retains moisture for a very long time and also inhibits the growth of mold. Peat moss is also very good and available at garden stores. Dirt is okay, but wet dirt is much more prone to mold and uninvited bug-visitors usually show up pretty quickly. Dried leaves from hardwood trees (like oak) are the very best substrate since they are a favorite food for millipedes. If the leaves are too deep, however, it may be difficult to see your pets. Consider crushing them up and mixing them into one of the other substrate varieties as a leaves-only substrate does not hold moisture quite as well.
Interestingly, portions of some millipedes' bodies fluoresce under blacklight. This is particularly common in polydesmid flat millipedes as seen in the video below.
Ventilation: All pets that are being housed in containers need fresh air. A little bit of air flow goes a long way in preventing the kind of habitat that promotes the growth of mold and even small mites that occasionally infest food that is left too long in millipede cages. Ventilation is achieved simply by poking holes in a container or by cutting out a section of the lid and gluing a bit of plastic or metal screen over the opening. Millipedes do not need much ventilation though, and desiccation (drying out) is the most common cause of death in captivity.
Humidity: Humidity is doubly important for pet bugs. Millipedes like to drink water and they need a bit of environmental humidity in their habitat when the time comes for them to shed their skin. Luckily, millipedes have fewer problems molting than most other bugs. Water is offered to millipedes in two common methods. A small, shallow water dish is preferred. A deep water dish may result in drown bugs. The addition of a few pebbles will help prevent drowning. If you don't want to spend the couple bucks for a nice looking water dish, any upside-down lid will do. Alternatively, you can just pour water into one corner of the cage every few days. The substrate will absorb most of it, but your millipedes will still be able to drink from it and benefit from the humidity that it helps their habitat to maintain.
Temperature: Room temperature is just find for raising all pet millipede species, whether nymphs or adults. As with most bugs, warmer temperatures and good food supply means faster growth. Cold temperatures lower metabolism and the millipedes will grow much more slowly.
Food: One of the really great conveniences about keeping millipedes as pets is how easy they are to feed. Millipedes are omnivores, so they eat pretty much anything and everything. Everything from fruits and vegetables to bits of dried pet food are eaten. One of the fun parts about keeping millipedes is that you can experiment with offering them leftovers from your dinner table. The primary diet of millipedes will be their substrate, including decomposing wood from hardwood trees, and leaves of same (like oak).
No millipede caresheet would be complete without some mention of Archispirostreptus gigas, the African Giant Black or AGB millipede. This once popular hobby species is nolonger imported into the US on account of the commensal (symbiotic) mites that live on them in their native habitat, and which proved to be damaging to certain agricultural crops in countries where they were imported as pets. Captive bred specimens occasionally surface in the US hobby, but this species was never a very good reproducer in captivity. They lived upwards of a decade and care was simple, but alas we speak of them mostly in the past tense now. Fortunately, captive bred specimens do not carry the commensal mites as this long dead specimen from my video below "naturally" did.