Just buy some Frontline Plus, apply it to your cat’s neck in a spot she can’t lick, and done.
It’s a topical poison, harmless to animals, but it kills of parasites very effectively. I’ve used it initially to treat an acute infection (found 2 fleas), and now use it as a preventive measure.
Per instructions you should use it monthly, but every 3-4 months seems to work just fine for me.