HI
I have usually found that keeping the background soundtrack at a constant lower level than the narration is somewhat better than dropping the volume every time the voice comes in. The latter makes it sound like there is automatic gain on the soundtrack which can be annoying particularly if blocks of narration are close together. But if that is what you want Borate's method (and yes, I have used it :-)) ) will work.
So, record your narration live while watching your video using the RECORD option. It will appear on Audio track 2. Left click the loudspeaker icon on Audio track 1 and then slide the general volume down to a suitable level e.g. 25 - 30%. Play the video and then adjust the value so the narration comes over clearly.
Live narration in this way can be a bit of a pain. It's easy to "err!" all the time or say the wrong thing, fluff your words or get behind with the visuals. .....Alternatively you could watch the video and write a script that describes the action and which has the correct timing. Record it separately as short sound clips. Put the clips onto Audio track 2 and move them into position to match the visuals. Now adjust Audio track 1 so your narration comes over OK..
Nat