Record the video while narrating (but not recording, i.e. don’t use a mic). Then import the video into Cubase and re-narrate the video with a microphone and your audio interface. You’ll note only be able to focus more on each aspect of what you’re doing, but the audio should be sufficiently high quality that you can use the Audio > Advanced > Detect Silence function to kill the stuff in between.
If you want to really get fancy, go find some downloaded cinematic tracks and incorporate those into yet another audio track in the Cubase project so that you have some good background music.