A wing divider will allow a nicer stitch when doing a saddle stitch. A channel keeps it from being slanted. I cut one piece to size and I cut any overlapping pieces slightly bigger then glue and trim to size. I find this results in a straight edge that needs very little sanding. I use goat leather which is difficult to mark, I will run my wing dividers around the perimeter then I often have to do it again, an inch or so at a time then prick, then again and so on. Often I will plant the first tooth on the line then lower the last tooth and press gently leaving a slight tooth depression to make sure I'm lined up. A rotary cutter works better on thin stretchy leather than a knife. I make my pattern out of thick cardstock then use double-sided tape to stick it down and use that as my guide for the rotary cutter.