no worries
The black line I just select the pencil tool (which is the same as the airbrush one, so hold down the button to get the pencil tool), use the color black (or any other color)...and then I usually zoom in pretty close to make this easier, but... You just draw a line around the edges basically. Hold down shift to make a straight line - it makes it easier if you don't have a steady hand. lol.
There's also another way to do it that involves enlarging the canvas size, but I just prefer drawing the line...I think it's pretty easy