What you are actually growing
A new filter is just wet plastic. The cycle is the process of colonising it with bacteria that first convert ammonia, which is highly toxic to fish, into nitrite, which is also toxic, and then a second group that converts nitrite into far safer nitrate. In a fish-in cycle, a living fish supplies that ammonia and endures it. Fishless cycling supplies the ammonia from a bottle or from decomposing food instead, so the bacteria mature in an empty tank and the first fish move into water that is already safe.
Any plain ammonia source works if it is free of perfumes and detergents; some keepers use fish food or a prawn left to decay instead, which is slower to control but equally feeds the process.
Setting up and the first dose
Assemble everything first: substrate, decor, filter running, heater set a little warm, because bacteria multiply faster in warm water. Fill with conditioned water, since chlorine would work against the colony you are trying to grow. Add your ammonia source in a measured amount, then test to learn what that dose produced, rather than trusting a recipe written for a different tank size.
From here the routine is test, note, wait. Test every day or two and write the numbers down rather than holding them in your head, because the pattern across days is the whole story.
Reading the three acts of the cycle
First, ammonia sits high and nothing else moves; the first bacterial group is still tiny. Then ammonia begins to fall and nitrite appears and climbs, often sharply. This nitrite peak is the stage where beginners lose patience, because it can sit stubbornly for days while the second group of bacteria, slower to establish, catches up.
Finally nitrite falls away and nitrate, the end product, accumulates. When you can add a dose of ammonia and find both ammonia and nitrite back at zero a day later, the filter is processing waste at the speed a stocked tank demands. That is the finish line, and it commonly takes several weeks to reach honestly.
Mistakes that stall or fake the result
Cleaning the filter, swapping its media or letting it dry out mid-cycle throws away the colony you have grown, so leave the filter entirely alone except to confirm it is running. Overdosing ammonia heavily can also slow things, since extreme levels inhibit the very bacteria you want. And a single zero reading proves nothing: the test that counts is a dosed tank clearing fully within a day, repeated once to be sure.
Do a large water change at the end to bring the accumulated nitrate down, let the water settle, and only then add fish, stocking gradually so the colony can grow with the load.
Recording every test in the tank's Pet Passport on KinsLe gives you that dose-and-clear evidence in one place, ready to check again whenever you upgrade the filter or start a second tank.