It depends on whether you want to do a fishless cycle or a plant cycle.
With a plant cycle, you do not add ammonia or bacteria. With this method the tank needs to be heavily planted with fast growing plants. A couple of slow growers are not enough. The tank is set up and the plants and other decor added. Fertiliser can be added. Then it's a good idea to take a photo of the tank. Now wait. After a couple of weeks, compare that photo to the tank. Have the plants grown? Are they all thriving or have any died? If they are all thriving and have sent out a lot of new growth, the first batch of fish can be added, and it is sensible to test for ammonia and nitrite every day until you are sure they are staying at zero. New batches of fish can be added every few weeks, testing after each batch, until the tank is fully stocked.
Some bacteria will grow slowly in the background but not nearly as many as in a tank with no live plants.
If only a few plants or slow growing plants are intended, then a fishless cycle is better. Some plants can't tolerate high levels of ammonia so it is better to leave them out until after the cycle has finished.
Ammonia can be added as soon as the tank is up and running. Safe Start can be added at the same time - many people pour it into the filter.
Ammonia is getting more difficult to find in the UK. Both eBay and Amazon sell it, but care must be taken to read the ingredients. There should be only ammonia or ammonium hydroxide and water. If there are any detergents, surfactants or perfume, it's not suitable. Many listings don't give the ingredients, so I would ignore those - better safe than sorry. The alternative if Dr Tim's Ammonium Chloride. Because it's a different chemical the calculator on here can't be used to work out how much to add, but the bottle does give a dose rate. Add what it says (so many drops per American gallon), wait half an hour for it to mix in then test for ammonia. If it's around 3 ppm, that's fine. If it's lower add a bit more, but make sure you add together all the drops so you'll know how much to add next time.
(fishless)
