Seperate males from females. Males are usually slimmer and more stream-lined than females, especially apparent when females are full of eggs. Condition them seperately on a diet of live black worms, bloodworms, live brine shrimp, daphnia, and a high quality pellt (lots use Hikari's Micro-pellets). In about two weeks, the female will be full of eggs. Transfer the conditioned pair to a tank (5g or 10gs work well) setup for spawning. Provide a Killie-fish-like spawning mop of soaked yarn (explained below), and provide plenty of clumps of Java Moss about the tank. filter with a matured sponge, and DO NOT FEED in the spawning tank. Keep it as clean as possible. Temp at 82, a very very low pH of <6, and almost NO measureable hardness is usually nessecary to spawn Cardinals. Keep the lighting dim, and then turn it off. The next morning, they should spawn at around sunrise. Check the Java Moss and Spawning mops for eggs. Alot of people also use a substrate of a double-layer of marbles, so the eggs fall between the marbles where the parents cannot eat them (which they will be trying to do). When you spot eggs, remove the parents back to whereever you'd like them. This has also been done with groups of 2 males and 4 females, or other equal ratios. Fry have to be fed on extremely small foods. Hence the matured sponge. Most are loaded with Rotifers and other small things for the fry to munch on. Supplement this with microworms and commerical fry food, and/or liquid fry food for the first couple of days. P. Axelrodi fry grow extremely slow, so be prepared to wait while they grow. You can also keep the Java Moss in there for food. Fry like to nip at things we can't see for food, and I know lots of times Java Moss is filled with tasty little creatures.
By the way... $6 for Cardinal tetras?! They're $1.99 around here!