some people use laser levels, others use chalk lines, although both of these can have issues with R/D designs. your best bet is probably a flexible rule, then use marks at center every 6 inches or so to mark center between. also a yardstick works pretty well, its what i use along with spring clamps to hold it in place with the contours of the bow. you could break the layout down into smaller increments and use point to point, it works well if you have alot of curves or other things that keep you from laying it out all at once.