I am building this kit, and I am a little farther then you.

In the directions on page 18, steps 4 and 5 show decent pics of what is needed. You have to add an inch or so length to make the sheeting long enough(pic 5 shows this). Yes, the ledge at the bottom will stick out past the sheeting and get sanded later. In pic 2(on page 18) you can see the sheeting butted up against the lower ledge and the sheeting ending in the middle of a stringer.

I hope this helped you, kind of difficult to describe