iMovie audio query

I have a video clip that I recorded with the Reflector application. The video clip is a short guide on how to do a particular thing on an iPhone.

I'm wanting to add a voice over whilst the video is playing in iMovie. But I'm struggling to be able to speak at the correct pace so that the voice over is at the correct point in the video clip for what I'm speaking (if that makes sense).

Is there any way that I can control the audio so that its at the correct point in the clip?