Venisonburger has it down pat. If you follow his diagram it's hard to mess up. A 5/32" chainsaw file dragged slowly backward while using your thumb as a guide will give you a line to sight down. If you make a straight shallow groove across the back, slightly rounding to the diagonal on the limb sides, you can sight from the belly and make sure that they line up. By removing small amounts with the file from side to side, you can make minor corrections, and come out even. Takes practice, but the small diameter file will cut any wood, and give a nice finish that fits the string perfectly.