I would scoot my 6 x 24" ruler along the selvage cutting a straight line as I go--keeping the ruler as absolutely straight and as close to parallel with the selvage as possible until I reach a couple inches past the maximum length I need. Since you're wanting an 8" wide strip--I would then have to switch to my 15" square ruler and line it up on the 8" line with my now straight cut edge and then scoot it down, cutting 12" or so at a time.
Personally, unless this was some kind of large print that absolutely had to match, I would cut it width of fabric & piece the borders. With a smaller print, you can usually line it up well enough to piece it without having to go through all the scooting & snipping.