Here is a very useful article for treating ick.
The first thing you want to do once you spot Ich is get the fish out of the water he is currently in. This could mean setting up a "hospital tank" with clean water that has been treated with a water conditioner to ensure the best water conditions for your fish while he heals. If you don't have a hospital tank, then perform a complete water change, clean your tank thoroughly, and add water conditioner before returning your fish to it. Remember that Ich spend three days swimming in the tank water, so putting your betta in new water will get him away from any free swimming parasites. The water in this new tank should be slowly raised up to a maximum of 82 degrees Fahrenheit (27.7 degrees Celsius). Bettas are tropical fish, so they prefer warm water anyway, but Ich thrive in colder waters so by raising the temperature you give the pesky parasites a lower chance of survival.
The next thing you should do is select a medicine to put in the tank that will kill the remaining Ich. There are three basic types of medications for betta fish: antiparasitics, antifungals, and antibacterials. Ich is a parasite, so if you're browsing the pet store shelves, look for a medicine that's advertised as an antiparasitic. There are a lot of medications that claim to kill Ich, but what you're looking for is one with either malachite green or methylene blue as a main ingredient. Check the label for one of these two ingredients before you buy the medicine.
Follow the instructions on the bottle very carefully and completely. Usually you just need to measure out the liquid in a syringe and squirt it into the water, although smaller bottles may have a built-in dropper to make it easier. If you would prefer to use a dropper, QuICK Cure by Aquarium Products is a treatment with malachite green that I have had a lot of luck with. The medicine won't be able to kill the Ich while it's attached to your betta, so you must keep using the medicine even if you don't see Ich on your fish. When the Ich are free-swimming, the medicine will take its effect and kill them. This usually means giving the medicine every day for a week, and it is a good idea to continue using it for several days after that week just to be sure that all the Ich are killed. It is a good idea to perform 25% water changes every day while you are treating your betta. Malachite green and methylene blue won't harm your betta, but they could be harmful to any tankmates such as catfish or frogs. You should only treat your betta if he is isolated. In addition, if your betta tank has a carbon filter, you will need to remove the carbon before beginning treatment with the medicine.The instructions that come with the medication can tell you more about what it will effect.