Make sure the window is level first. If its not and you level the window sill, it might be noticeable right away. It depends on how much.
If it is ok, then place a long level on the surface and check its bubble. Then raise the right or left side to center the bubble. Use a pencil and mark a line on the window frame indicating the spot where you need to add some shims.
Do not use shims that are about 1-1/2 inch wide. I have seen shims that are about 4 to 6 inches wide. use these instead.
Then lay your windowsil on it and add more shims to support it.
Before you secure the windowsil, place your level back on it and check to see if it is still level.
If it is ok, then place a long level on the surface and check its bubble. Then raise the right or left side to center the bubble. Use a pencil and mark a line on the window frame indicating the spot where you need to add some shims.
Do not use shims that are about 1-1/2 inch wide. I have seen shims that are about 4 to 6 inches wide. use these instead.
Then lay your windowsil on it and add more shims to support it.
Before you secure the windowsil, place your level back on it and check to see if it is still level.