This is how I install soffit J channel. You can’t see it in the video, but there is a line that I’m following to staple it on. I don’t bother using a tape measure as it’s much quicker just to hold it up and go straight to the cut. You may ask why we don’t use F channel here, it’s because it isn’t available. I’ve been doing this for over 20 years and have never seen F channel in my life. Our suppliers don’t bring it in. This is how it’s done in my region - it works. We get wind storms over 60 mph