How can I prepare for an internal promotion interview?

[Boxingshibes]

I have applied for a role a grade above me. I am actually doing the grade up job.
I really need the pay for the grade up to £10k more.
I've been asked to attend a 3rd stage interview. Jumped to 3 as they know me.
How do i not fuck it up?

Is there anyone who can help you practice? What's the industry?

I would advise you to treat it as though it is an external interview, and make your written application as full and formal as usual.
(When I applied for an interview in similar circumstances and was successful, I was later told that several other candidates had relied on being already known to the interviewers and had not entered well completed applications, nor answered interview questions as fully as they might, and had been deselected accordingly)
I would also advise you to rehearse out loud. Tell the cat what you know, tell your partner, tell the houseplant. This gets you used to delivering your answers confidently.
Always remember that you can ask to return to questions that you think you may have more to give, and say this is why. Similarly, if you screw it up, say so and ask to begin again.
Good luck!