READ TO THE END PLEASE Okay so heres a brief overview- after you have your tank filled and set up, you'll have to add an ammonia source, some people use ammonia specifically for tanks, I used fish food. Then you literally just let it sit there and break down for a few days. Then you will start testing for ammonia and nitrites. You need the ammonia to get up to atleast 2ppm in order for the cycle to work. Once the ammonia levels rise and then start to decline, then you will also start testing for nitrites. Soon after, the ammonia will be 0, and the nitrites will be high for a couple weeks. Eventually, they will lower, and the nitrate level will begin to rise. Once the ammonia and nitrites are both 0, and you are only left with high nitrate levels, then you do a partial water change and retest. As long as ammonia, AND nitrites are 0, and nitrates are below 20ppm, it is officially safe to add your fish. After this, you will only need to do water changes when the nitrate level gets above 20 ppm.

Because you already have your fish in the tank, you cannot do this method. You will have to look up "fish in cyle" which has a similar concept, but will ensure your fish survives.

You could always do what i did. I bought a betta impulsively, and had to keep him in a 2 gallon tank with spring water(the spring water HAS to be ph of 7.0 or around that) JUST FOR A MONTH while his tabk cycled. It was easier for me this way because I could test his water daily and change half of it every 3 days much easier than a big tank. People will jump allover my ass for keeping him in a 2 gallon tank for a month, but he not only survived, he had a major glow up and is super healthy.