The way I like to treat mild cases of ich with Bettas is the two cup method, I use two small container and switch them out each day from one cup to the other, cleaning the cup between changes making 100% water changes. I do this for at least 3 days past the last day I could see the parasite on the fish.
You can't kill the parasite while it is on the fish and you can speed up the parasites life cycle with higher temps. In temps over 86F the parasite is reported to stop reproducing.
Aquarium salt 1tsp/gal can also help kill the parasites if they are not resistant to the salt and some are due to over use and wrong dosage of salt over the years. Salt can also help the fish by helping them make a heavy slim coat and avoid a secondary infection due to the wounds from the parasite after they drop from the fish.
These parasite can live in the substrate so it is a good idea to make deep gravel vacuuming to manually remove them.
You can have ich in the tank and it will not attach to the fish unless the fish becomes stressed from poor water quality, overcrowding, other illness, sudden temp changes...etc.......sometimes just fixing the problem in mild case the ich will go away without treatment.
Often the ich can attach in the gill area and you can't see them, but you will know when the fish flashes (rubs/itches) his gill area on objects in the tank, other things can cause this as well...not just ich.
In server case ich can kill the fish so it is important to start treatment as soon as you can and then fix the problem to prevent it..i.e. poor water quality-step up regular water changes, re-eval stocking, keep temp stable etc.....