The easy way to cycle a tank is to just plant it heavily, turn on the lights, filter, etc. and start your fertilizing regime. Wait a couple of weeks, making sure the plants have started growing well, then add a few fish, maybe a couple for a 20 gallon tank. Wait another week or so and add a few more, maybe 3-4 more. Wait another week or so, and you should be able to finish adding all of the fish you want. The growing plants will be consuming most or all of the ammonia from the fish, but a bacteria colony will also be developing. You will have given it a month or more to fully develop, which should be plenty. And, you will not have forced the tank to grow an oversized colony of bacteria just to die for lack of ammonia when you finally stop dosing ammonia. This has always worked for me.
But, if you are going to buy $10 apiece fish, or rare ones, or special order a school of 50 cardinal tetra, etc. then you will want to use the conventional ammonia dosing method for establishing a big enough colony of bacteria to be absolutely sure you won't be in any danger of harming your investment in fish when you add them. This is how I see it anyway.