What is the easiest way to organize voice recording over video?

Question

YouTube

Hello. Please advise how best to organize (faster and better) voice recording on video.
Essence:
1) There is a video on Youtube (40 min)
2) You need to turn off the native sound on it.
3) Write down your comments on top.
4) The video will not go in real time, but there will be stops, small rewinds.
What's the best way to do it here? What software/services to use? So that it could be easily uploaded to YouTube (or maybe you can do everything on YouTube at all. I don’t know honestly).
Please push me in the right direction :)