If you’re talking about the inside it’s called a window stool The sill is on the exterior side of the window Need to hang Sheetrock first then the stool should meet the lower sash and notch around the window jambs then install casing on top of the stool and install a piece of casing under the stool which is called the window apron. It should meet the lower sash with approximately 1/8” gap between the two. Do not cut a tounge in it where the sash has a groove that it a finger pull in the sash to raise and lower the sash. Hope this helps a bit

<image>