Nitrifying bacteria do not like ammonia or nitrite over 5 ppm, but do not seem to care about nitrate.
They require carbonates, they use that as a source of carbon. If the KH can be held stable as low as 3 degrees is fine, but a lot higher works, too. If it keeps dropping (for example, I have substrate that keeps removing the carbonates) keep adding carbonates.
Once a day (not more often) add enough ammonia to read 3 ppm.
When the bacteria can remove the ammonia within 24 hours and nitrites also read 0 ppm the cycle is done.
If you are still not ready for fish keep on feeding the bacteria.
Water changes are optional, but when you are ready for fish you will need to do a really large water change to get the nitrates way down.