You may use the following videos as examples of natural dialogue to practice, or you may find your own examples of natural speech. You can also use movies in which you really feel the dialogue is well written and performed with excellence.
Some of these videos are long. You do NOT need to use the whole video. Feel free to start somewhere in the middle and use 60-90 seconds for practice.
If you don’t use the whole thing, don’t start at the beginning. Pick a spot in the middle. Generally, people become more comfortable and things become more natural as the video goes on.
Donald Trump and Nancy Pelosi bicker: https://www.youtube.com/watch?v=6wm-NtMkqz8
How to Become a Butcher: https://www.youtube.com/watch?v=ZD26VeYEdEU
Man Sees for First Time in 35 Years: https://www.youtube.com/watch?v=GY5Q13Q2Q_4
Scrivener is the only app I recommend. You really don’t need any other software or apps for writing a book than this one.
Find it here: https://www.literatureandlatte.com/scrivener/overview
Their 30-day free trial allows 30 days used. If you don’t use it every day, it will last longer than a month.