The sill of the window is simply the bottom of the window frame. A window sill can exist in one of four states: finished drywall, unfinished with the window framing studs exposed, finished using a nice window sill material or finished with old, deteriorating window sill finishing material. A new window sill is very easy to install, but before installing a window sill, the sill area has to be measured so the window sill material can be sized to fit the area correctly.
Remove the old window sill material and all associated debris from the window sill area, if required. Use a vacuum or a shop vac to be sure all small bits of debris are removed.
Stretch a tape measure from the window frame to the edge of the wall to find the depth of the window sill.
Stretch a tape measure from one side of the window opening to the other to measure the length of the window sill.