Great info at www.monarch-butterfly.com
Monarchs are in trouble in the US. Monarch caterpillars in the wild have only a 5-10% chance of maturing into butterflies. Collected caterpillars raised in containers have a 95% chance of maturing into butterflies (who can then breed and make more!). I use a sterlized white 5-gallon plastic bucket (food grade) with a screen lid (actually a grease spatter screen from Dollar Tree). I put a small twiggy branch in it because when the cats get big they instinctively climb up to find a spot where they can go into the chrysalis stage. They eat ONLY milkweed leaves. I usually put in one fresh good size leave per cat per day. I try to limit the number of caterpillars per bucket to no more than 4-5. They need to be protected from birds, being stepped on, predatory wasps, other insects, severe weather, etc. One of the biggest killers of eggs, caterpillars, and chrysalis is roadside mowing machines.
It is DEAD EASY to raise them. Just set the enclosure where it gets indirect sunlight (I keep mine on a table in the office) and put in the milkweed leaves every day. They don't need anything else. Once every couple days, I remove the dried up chewed up leaves and as much of the droppings as I can. They aren't very active, so I usually lift out any cats that are on leaves, set them on the table, do the housekeeping and put them back. I don't touch the cats...not only are they "icky" but you can transfer germs to them.
You can get very elaborate with a monarch raising setup, a friend of mine converted a small chifferobe/Armoire into a big monarch cabinet complete with lighting, but it's really not necessary. She travels the county collecting eggs and cats and usually releases upward of 100 butterflies every summer. My record is around 20.
I really REALLY encourage you to try it. It makes a great project for kids and it's so rewarding to watch the butterflies emerge (especially if you catch them in the act of transformation...from caterpillar to chrysalis (after 2-4 weeks depending on size) and from chrysalis to butterfly (about 1 week).