Easy. Just hit the rec button, select narration... change settings and record. The audio will be placed on a new track by iteself.
As for the video of you in a corner. Grab that file and drag it to line 3 on the timeline (if you had reg video on line 1, narration on line 2. Then hit effects and change size position (while portion of video that is going to corner is lit in yello) the resize and place