How to Repair a Rotted Windowsill

Make a plumb cut flush with the sheathing along the length of the rotten sill. A circular saw with a beveled blade can handle most of the cutting, but for the sill ends where a circ saw can't reach, switch to a multitool, like this Fein Multimaster equipped with an oscillating saw blade. Smooth the cut with a few strokes of a hand plane.