Windowsills are the horizontal pieces of wood at the bottom of windows that provide a ledge. In older homes, the sill may actually be a physical part of the window. In that case, the sill is the board that the window closes down on. In modern homes, most windows have integral and energy-efficient window sills that are part of the window construction and do not project into the room. In those instances, most builders have built a wooden sill that will butt against the manufactured portion of the window. Windowsills need to be refinished or repainted fairly often due to damage from constant sunlight and moisture from open windows. This is usually an easy project that can be done in a few hours at most.
Tape the trim moldings or plaster that meet with the windowsill. Tape the bottom of the actual window (any portion of the window that touches the windowsill that isn't made from the same piece of wood as the windowsill). Cover at least two inches with tape on the sides. Tape off the wall as well where the sill and wall meet.
Lay the paper bags along the floor under your window so you don't make a mess.
Sand the windowsill with your sanding block, starting with the coarse block if you are removing old varnish. Go from coarse to medium to fine grit as the old finish or problems with the wood are addressed. Wipe down the wood between sandings. Fill any holes with wood filler. Let it cure, and then sand those spots to make the surface as smooth as possible.
Use your screwdriver and bits of sandpaper to sand the tight edges. (Don't miss any of the old finish. If you do, it can look yellow when you reseal your finish.) When you are finished, wipe down the entire windowsill making sure it is very clean. Any debris you leave on the sill may become lodged in your sealer.
Shake the stain well. Put on protective gloves and open and apply the stain. Wipe it on in the same direction as the wood grain. The old stain should still be present, but it may have light or dark spots from water or wear. Apply it evenly and wipe off the excess.
Allow the stain to dry thoroughly (see the manufacturer's recommendation). Mix the sealer, and apply one or two coats as recommended. Allow the sealer to dry between coats. When you are finished and your windowsill is dry, remove the tape and discard your waste.