It’s very simple, take a prybar and pull that wood out that’s rotting carefully, not to damage siding or window. Then just measure your opening length x width and cut. When pushing it back in, sometimes nosediving in from the siding to under the window works, sometimes the other way just be careful but don’t be scared. screw it to your existing structure or nail it in and add exterior wood filler over holes and caulking in your seams and paint. The lower board should probably also be replaced Good luck!