Hmmm, from the top of my head....Cardinal Tetra are egg scatters.....Go purchase some grass mops...I think that's the term, "grass mops"...It's a breeding aid that resembles a mop head....When breeding, the males will chase the females through the mops....The eggs and sperm will fall into the mop and be fertilized....I believe the tetra eat their eggs, so simply take out the mop and place it into a hathery tank....
Cardinals are extremely difficult to breed in aquariums. Try neons for the experience first but even though most of them are being bred in Asia they remain a real challenge to breed and cardinals are about 10X more difficult. Even if you have breed easier tetras like black Neons or Black phantoms they do not prepare you for the challenge of raising cardinals.
As apistomaster already mentioned, neons are MUCH easier to breed. You have better chances of getting a captive raised pair of fish with neons also which is helpful.
The key is to start feeding your fish quality live foods from a young age. Flake isn't the answer. Variety and quality of live food is. Water quality is also a key factor. PH should be low but hardness is the key thing here. It needs to be super low.
I learned this from a crusty old dude that has bred just about everything. Hope this helps