You should prepare in the same way that you would for any interview, but breathe easy, as an informal interview is exactly that!
They are increasingly popular over competency based interviews (although may still feature competency based questions – “tell me about a time you’ve dealt with something difficult” or formal questions about what you know about their company, so prepare thoroughly) but are held in a more conversational style.
It’s a little easier to explore the opportunity and company you’re applying to, you’ll have more flexibility in asking questions instead of saving them all for the end, and these can be a really good opportunity to see if the job and team are a good fit for you.
As long as you make sure you speak clearly and remember to keep your body language open – make eye contact with the interviewer, don’t fold your arms, try not to slouch – the conversation can flow as if you were speaking to a family friend.
You should still go “dressed for the job you want”, but you can relax in the knowledge that this won’t be a quick-fire-questions round.