Open the window and remove the screen, if possible. Brush or vacuum the loose filth -bugs, cobwebs, etc Vacuum these up and try not to squeal or make other silly noises if any of the bugs happen to decide they are alive. Once all of the loose dirt has been removed dip your brush into the soapy water, scrub the track, rinse the brush, and wipe away the soapy reside with the damp rag. You’ll need to frequently rinse the rag, soap residue is sticky so don’t skip this step or the track will be an even better dirt magnet. Replace the screen and congratulate yourself for getting around to the job after X years.