Window sills are designed to divert water away from the house and as a result they get a battering from the weather, over time wooden sills can rot and concrete window sills can develop cracks. Cracks in concrete window sills can be caused by the freezing and thawing of water seeping into tiny holes in the concrete, this can also cause the corrosion of the steel reinforcing bars inside the sill which could also crack the concrete. It can however be the result of subsidence. If you suspect the cracked window sill may be due to subsidence it would be worth getting it looked at by a professional. If the crack keeps reappearing or is increasing in size this could be an indication of subsidence.
Rub off flaking paint or loose concrete along the crack with the wire brush and make sure to clean out any moss that may be growing inside; as this may prevent the filler from making a good seal. Remove any dust with a soft brush.
Fill the crack with Multi Purpose Exterior filler. Depending on the type of filler you have purchased application may be via a nozzle; or you may need to use the filling knife to mix and apply the filler. Fill the crack slightly proud of the finished level then smooth out with a wet filling knife. Try to avoid this type of repair when it is raining as the filler may get washed out or saturated with water which will cause it to lose its strength.
Sand down the filler when it is completely dry. If the sill is plain concrete it will not need to be painted; however you should be able to purchase grey exterior filler which will be a close match to the concrete. When sanding; dust from the sill will rub into the filler making it dirty but also blending it in with the rest of the sill. If the sill is painted repaint the whole sill as this will give an overall better finish.
When painting concrete window sills use an exterior masonry paint.
If working off a ladder use a Builders tool belt to carry tools and materials, this will help to keep hands free when climbing the ladder.