For the basic method, you only need cleaning vinegar or plain white vinegar, an old cloth or a few sheets of toilet paper, toilet gloves, and a toilet brush. Start by flushing the toilet so the bowl is wet but not filled with loose dirt. If the stain is high on the side of the bowl, you may want to gently press away some excess water first.
Soak the cloth with vinegar. It should be wet, but not dripping all over the floor. Then place it directly on the stained or chalky area inside the toilet bowl. If the mark sits around the waterline, press the cloth against that ring. For smaller patches, fold the cloth so it covers the exact spot. The goal is simple: keep the vinegar touching the stain.
Now leave it alone. That is the part many people skip, but matters most. Give the vinegar at least an hour to work. For stubborn buildup, leaving it overnight is often more convenient, as long as nobody needs to use the toilet during that time. Close the lid, leave the room ventilated, and let patience do the heavy lifting.