A few years ago I tried multiple times to make a screen for my kid’s bedroom and failed each time in a different way. I got so angry I gave up. Each time something doesn’t line up or fit correctly even after checking the dimensions multiple times on each try.
I made screens as a job in high school. Make sure you make the screen against two square edges but the most important part is to not over tension the screen material when putting it in as it is strong enough to pull your frame way out of square. A slightly loose screen material is better than an out of square frame.
As for making sure the frame fits I don’t have great advice other than measure twice and cut once and make sure you take any corner connecting pieces into account in your measurements.