Load your project, then click the NARRATION button on the Home menu toolbar. 
To begin to record sound, first navigate using the preview timeline to the place where you would like to begin recording. Once you are happy with the location, click the Record button or press the spacebar and begin recording your audio. To Pause the recording, click the Pause button or press the spacebar.
Once you have finished recording, click the Stop button. Your recorded audio will be automatically added to your sequence in the exact location that you chose to begin recording. If you would like to record sound in various different places in your project, you can repeat the process, or Record and Stop as many times as you like. Just position the red-lined cursor where you want the audio to appear.
Sound clips can be dragged right or left on the audio track, should they need to be lined up more precisely.