What do I say about why I left my last job?

I have an interview next week and I am unsure how to explain why I left my last job as a bookkeeper. This is my first time dealing with any of this. I have been practising in the mirror. What do you say?

Say it plainly and move on to what you can do. Interviewers usually move on if you do.

Practise it out loud with a friend. Mine felt natural after the third try.