Short answer: the fastest way to sound natural is shadowing: repeating a recording half a beat behind it, copying rhythm and pitch as well as words. Combine it with a small set of fillers (えーと, あのう) that give you time to think, and a few prepared scripts, like a self-introduction, that you can say without effort.
Japanese speakers pause with sounds and set phrases. Use them instead of English “um”.
Say it until it is automatic; then change the details.
Answer each question aloud before pressing play on the model answer.
Frames for saying what you think, from soft to firm.
A four-line anecdote to practise aloud.
Try each one, then open the answer to check and hear it.
Phrases that keep a conversation going when words run out.