I have three wall sections each approx. 10'H x 5'W with a 4'x3' window in the middle. I've had no problems using the track on a simple section, but I'm stuck at how I'm going to approach this. My first thought is to loosely tack the fabric around the perimeter of the section and then carefully cut an 'x' for the window, but not all the way to the corners. Then start tacking in around the window cutting more into the corner as needed. I think the first 'tacking in' will have to be relatively loose, or else it's going to be too tight when I start going around the window. Sound similar to what you did? Tips?

That sounds about right. I used some fabricmate track on parts of my soffits and on parts of my stage. I ran into similar situations and found it best to leave it looser than I thought it should be and then keep working around taking up the slack little by little.