How to practise for an interview alone
Rehearsing in your head fails, because the hard part is speaking.

Reading your notes feels like preparation. It is not, because the skill being tested is talking, under mild pressure, in a way another person can follow.
Why silent preparation fails
Recall and production are different abilities. You can know a story completely and still deliver it as a shapeless three-minute ramble. The gap only closes by speaking.
A workable loop
Run this for a single question at a time:
- Answer cold. No notes, out loud, recorded.
- Listen back once. Note only the structural problems: no ending, buried result, three tangents.
- Fix one thing. Just one.
- Answer again. Cold, recorded, no rehearsing in between.
Two or three passes per question is usually enough. Past that you start memorising, which sounds worse than thinking.
Choosing questions
Prepare stories, not answers. Six solid stories, each with a situation, your specific actions, and a result you can quantify, will cover most behavioural questions asked of you.
Interviewers do not remember your phrasing. They remember whether they could follow what you did.
For the technical portion, practise narrating while you work. Silence reads as being stuck, even when you are making progress.