Females are the fat looking ones. Commercially they are bred in ponds.
I've heard of manipulating the photoperiod to make them spawn, but they don't breed readily in most aquariums and since they are only a dollar or two a fish (retail, a breeder would get a fraction of this price) few people bother to breed them. Also, they generally spawn at water temps around 70-72F which is a little colder than most people's tanks. They are egg scatterers that readily eat their own eggs and fry. Eggs will generally not develop if they are exposed to light and tiny fry will need infusoria for feeding (Too small for brine shrimp nauplii, etc..).
You really should set up a dedicated tank if you want to raise them. The chance of any surviving in a community aquarium is effectively 0%.
I used to breed all sorts of tetras one at a time just to see if I could and the best way I figured out how to do it was with a barebottom tank full of java moss, duckweed covering almost the whole surface, and adding blackwater extract I made from boiling oak leaves. I picked conditioned neons out of my 55 community tank that were healthy and active and the females were all really plump (gravid). I put them into the 10 gal (with water from the main tank) I was using for breeding, fed them a bunch on bloodworms, and when I was ready for them to spawn I added enough blackwater extract to give the water a slight yellow tint. The next morning there would be eggs all over the bottom of the tank and in the java moss. Then I'd throw the parents back in the main tank and just run a well seasoned sponge filter. The eggs would hatch by the next morning, wouldn't feed them the first day, and on the second day I would feed them boiled egg yolk mixed with water twice a day and let them pick at the infusoria in the moss. Within 10 days they were big enough to eat powdered flakes and once they got a little bigger I fed them cyclopeez until they were 1/2 inch long and could be put into the community tank.
I never did any water changes and just topped it off with RO water. Because of the duckweed, ammonia, nitrites, and nitrates were always zero.
A big difference between my setup and most is that my main tank had a pH of 8, was super hard, and had at least 1 tspn of salt per gallon.
This is totally counter intuitive from everything I ever read about them but I was lazy and didn't want to use a whole lot of RO water. My RO filter is an API Tap Water Purifier and is a PITA to use. I think the spawning trigger was the pH drop from 8 to 7 from the blackwater extract and the low light conditions. I bred serpaes, black neons, glowlights, black skirt tetras, flame tetras, lemon tetras, bleeding heart tetras, and probably a couple others with the exact same setup over a two year period. I'd trade the babies to the LFS for store credit and buy some new fish to try to breed. It was cool having different fish in the tank every month and kept down my urges to set up a dozen more tanks.