I own a "cookie cutter" house built in 2006, and all of the windows are the thin vinyl kind with no trim at all, but I like the look of having a window stool. Is there a standard practice for adding stools to this type of window? Do I need to remove the drywall on the bottom and attach the stool to the frame, or can I just place it on top of the drywall?
Removing the sheetrock will lower the finished stool height a bit which will help maintain a more even reveal on the window, and make it look less like the stool was added on. Cut around the edges first with a utility knife to minimize damage to the surrounding jambs & wall below and then just pry it up.
If you really want to keep the same reveal as the old sheetrock, you could build up your new stool out of 1/2" thick stock (assuming the sheetrock was 1/2"). Make one piece that is the size and shape of the entire stool, then thicken it where it projects out from the wall by laminating another 1/2" thick piece underneath.