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.