You will know they are ready to spawn when you can easily tell the slender males from the bulkier, larger females, visibly swollen with eggs. Wild-caught neons need very soft water to spawn. The tank-bred ones may be less picky about it, but the wild ones want it very soft. Peat does wonders for that.
Some books seem to imply that a coarse nylon floss is a good substitute.
I would not bother, since Java moss is good enough and easier to grow than nylon. Temperatures of 70-75F are sufficient. They are said to spawn best when the sunlight hits the tank in the morning. Neons are voracious egg eaters. Remove the adults within hours after spawning.