This is what I do at least once a year. Clean more often if you use a lot.
I fill the tub with water, at least luke warm, cleans better with warm or hot water. Angle the jets to a down position. Add about a cup of bleach and then run the tub for about 20 minutes. Drain. Fill with water. Run the jets for 5 minutes to get rid of any residual bleach. Drain, buff dry, that is it.
Don't be adding any soap that will be foaming up or you will be sorry

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And if I moved into a previously occupied house, I would do this before my first use. I am very fussy when it comes to bathrooms.