Actually, it's a dead simple, very easy job. IF YOU DO IT THE DEAD SIMPLE, EASY WAY.....

If you don't use a trim iron, it's a pain. Like trying to stuff your overweight Aunt through the back door of your sports sedan.

If you cut out long, slightly too wide, strips of covering and then crease that strip down the center, it fits down into the gap nicely. Don't crease it and it's like trying to..... it ain't easy at all. Crease it, and it's so easy you'll wonder why you thought it wouldn't be.

And after it's ironed into the gap, it's "overwideness" will be sticking out. Use a razor blade to trim the "flashing" off. That's also dead easy to do.

OK.... to sum up:
trim iron
crease it
razor blade