In the canvas area, click the 'a' button on the "Bus-marimba1" bus' control box, to open the "automation" menu.
Click 'Fader' in the automation menu.
An automation track, which controls the fader, will appear underneath the bus.
If you click in the automation track, a point will appear. Each point represents an absolute setting for the control. After the point appears, if you click-and-drag it, the yellow numbers by the cursor will tell you the fader's setting at that point.
If there are two or more points in the automation track, lines will appear to connect them. The fader will be moved gradually between absolute settings, as shown by the line connecting the points.
If you make a mistake and want to start over, you can press the 'clear' button on the automation track's control box. Unfortunately, you can't remove a single point. This isn't really necessary anyway; if you accidentally add too many points, simply use the extra one to keep a setting constant.
Add one point to the beginning of the automation track, with a setting of 0.0 dB
Add one point at about 00:00:15.000, with a setting of 0.0 dB
Add one point at about 00:00:16.500 (where the singer starts), with a setting of -10.0 dB, or whatever you set earlier.
Now you've set up an automation plan, but the fader is still in "Manual" mode, so the automation track will have no effect. Change the automation track's setting by clicking on the mode button in the track's control box. The button currently says "Manual."
From the menu, select "Play," which will cause the automation settings to be played. In "Manual" mode, you have to adjust all settings manually. In "Write" mode, changes that you make as the session plays will be recorded into the automation track, over-writing previous settings. In "Touch" mode, changes that you make as the session plays will be incorporated into the pre-existing automation settings.
Finally, listen to confirm that you like the automated panner change. If you don't, you can always adjust it now or later.