Take the glass you just removed from the frame and place it on top of your wallscroll. You'll notice that it is smaller than the wallscroll, but only by a few inches on all sides. Only the portion of the wall scroll directly beneath the glass will be visible once framed. Position the glass so that nothing important in the picture is clipped off.
Important: If you chose to keep the sample poster, make sure that the pictured side is face up. Otherwise the image on the poster will show through the wallscroll.