While I agree with the fact sheet that separating the sexes before breeding increases the egg output for the first spawn and increases interest in spawning, I wouldn't recommend it because the males can get VERY aggressive with the females to the point that sometimes the females can be fatally injured. Red Rainbowfish (G. incisus) are the most aggressive rainbowfish, so I REALLY wouldn't recommend the separating strategy with them. This species will give you plenty of eggs as it is, if the fish are in good condition. It's best to spare your females the extra stress and not get the males too riled up.
Anyway, about your question of how to separate the eggs from the adults. For this, a spawning mop is very useful. It's basically a mop of 100-200strands of yarn that are attached to a float, so that the yarn hangs down in the water. Rainbowfish will spawn in this preferentially and the mop will help hide the eggs from adult fish so they won't get eaten. Here is an article describing in great detail how to make one:
Condition your fish by feeding lots of live foods such as blackworms, daphnia, brine shrimp, etc or frozen blood worms for a couple of days, then add the mop to the tank (make sure that there is only one species of rainbowfish in the tank as rainbows are notorious for hybridizing and the hybrid offspring generally look very bland). You could also put the breeders (hopefully more than just a pair and more females than males, 2 males and 4 females should give excellent results) into a spawning tank separated from all the other fish, before you start conditioning. Continue conditioning feeding while the mop is in the tank for 7-10 days, then remove the mop to a hatching/breeding tank or if the fish are in a spawning tank, remove them and leave the mop in the spawning tank. Alternatively, you could wait until you start seeing eggs in the mop or fry start to appear (if you can spot them before the adults do that is!) to separate the adults and mop. That should get you started, good luck finding some females!