You are doing it right. After a few days you will see less spots on your fish, because the ich are dropping off into the substrate. You can speed up the life cycle by turning up the temp gradually to around 86 degrees, which will make them fall off your fish faster. The medication will kill the ich once it has fallen off the fish.
When the ich falls off, it lands in the substrate, creates a cyst, and then explodes into thousands of new ones, which will get back on your fish. So with the meds, you are killing the ich as it falls off, or is swimming back up. The reason you keep using the meds for a few days after you see the last spot is to make sure you have killed them all. You should do a deep gravel vacuum when this happens too.