Are you breeding for entertainment purposes only?
10-20g sized tank usually works best. A sump with live rock in it would work well. You won't need live rock in the breeder tank. Use some epoxy and some chopsticks/toothpicks to make a fake urchin. - you could go as far as painting it black to look like an urchin. Feed them some high quality home made foods and keep the water clean. It might take months until they start breeding - but when they do, it's very exciting.
Cardinals are mouth brooders - Once their young hatches, they look like little bitty cardinals right off the bat. They need to feel the protection of the urchins spines, so they hide in them. As far as food goes, I've heard of some people feeding very finely crushed up flake foods. I would, however, recommend finely grated home made foods instead. Cardinals aren't as difficult to raise from babies as demersal spawners are .. like clownfish larvae.
Most importantly - have fun with it - and find a mentor.
I hope that Bang Guy can come on this thread and gives some of his vast knowledge and experience on the subject.