What is the difference between practice and practise?

In British English, practice is the noun and practise is the verb. In American English, practice covers both and practise is not used.

She went to football practice. (noun) She needs to practise more. (verb, British) She needs to practice more. (verb, American)

The test that works for British writers: substitute advice and advise, which sound different and so make the part of speech obvious. If advice fits, use practice; if advise fits, use practise. The c-noun and s-verb pattern is the same in both pairs.

The same split affects licence and license in British English — licence the noun, license the verb. American English uses license for both. And defence and offence are British spellings of what Americans write as defense and offense, though those are not noun-verb pairs.

A related question is whether to write -ise or -ize — organise or organize. Both are correct in British English. -ize is not an Americanism; it reflects the Greek origin and has been used in Britain for centuries, and Oxford University Press prefers it. Most British newspapers and Cambridge University Press prefer -ise. American English uses -ize consistently.

A handful of words only ever take -ise regardless of variety, because the ending is not the Greek suffix at all: advertise, advise, supervise, surprise, televise, compromise.

Pick a variety and apply it consistently. Mixing them within a document is the visible error.

A parallel pair catches British writers out in a different way: program is the correct British spelling for computing, while programme is used for everything else — a television programme, a computer program.

The practical safeguard is to set your word processor's language to the variety you are writing in. Most mixed-spelling documents come from a default setting nobody changed.