1) Ammonia is kept below 3-4ppm (don't dump it into a fishless cycling tank with 8ppm ammonia or all the bacteria will just die)
2) The bottle is fresh (ie hasn't been sitting in the cold/hot temperatures or for a long time)
3) pH is >6.5 (ideally 7) (very low pH inhibits/slows bacteria reproduction)
It's the same stuff as the old Biospira, but a non-refrigerated mix that's slightly less effective.
Filter squeezings also work, and are free, but aren't always available/available in the amount we need.
Read the label.
You are looking for a product that contains Nitrospira species of bacteria.
The last time I looked the right species of bacteria could be found in:
Dr. Tim's One and Only
Tetra Safe Start
Microbe Lift's Nite Out II
Perhaps some others... read the fine print.
Instructions on most of them are similar:
If you already have a problem with ammonia, do a big enough water change to get the ammonia almost undetectable.
Some products MIGHT be sensitive to certain dechlorinators, but I have not had problems growing bacteria. I use Chloramine Buster.
The bacteria do much better when they have a few minerals, and carbonate. If you can, set up the tank with somewhat hard water while you are cycling it, then you can do a water change with softer water right before the fish arrive if your fish want softer water. The bacteria will slow down, but that is OK. You want the best possible conditions for the bacteria to reproduce and get going in the new set up.