Join Date: Jul 2009
Location: Channelview, Tx
Following the Toolman's thoughts, here is my rendition of what you may be looking at. The drawing is generic and not to any scale.
The typical wall framing is colored to look like OSB. The window sill is in place, and your new frame is inserted in the opening.
Once you get the frame installed, you can trim it out as needed. Most likely you will need filler strips like I have shown so you can attach drywall or plywood or whatever you choose to cover the remaining part of the opening.
Hope this helps, and doesn't add to the confusion.