The American rule: one spelling for everything
In American English, "practice" with a C is used for both the noun and the verb. You practice the piano (verb), and you have piano practice (noun). There is no need to think about "practise" at all if you write in American English.
This simplification happened naturally over time as American spelling conventions consolidated. If your audience is American, use "practice" everywhere and move on.
The British rule: C for nouns, S for verbs
In British, Australian, and most Commonwealth English, the distinction mirrors advice/advise. "Practice" with a C is the noun. "Practise" with an S is the verb. This is a firm rule in these dialects, not a preference.
- Noun: Football practice starts at four. The practice of law requires a licence.
- Verb: You need to practise your free throws. She practises medicine in London.
- Test: Replace with "preparation" (noun) or "rehearse" (verb) to check which you need.
Choosing the right form for your audience
The most important thing is knowing your audience. If you write for an American readership, always use "practice." If you write for a British, Australian, or international readership, maintain the noun/verb split. If your audience is mixed, pick one convention and apply it consistently throughout your document.
Style guides are unanimous on one point: inconsistency is worse than either choice. Switching between "practice" and "practise" without a clear pattern signals carelessness rather than regional awareness.