The best way is to keep it simple and timely. Inform your manager as early as possible using whatever method your workplace usually prefers (call or message for urgent notice, email for formality). A quick message like “Hi, I’m not feeling well today and won’t be able to work. I’ll keep you posted on tomorrow” is enough; no need to overshare details. I usually add if anything urgent needs coverage just to be considerate, but overall, short, clear, and respectful works best.