1x4 piece mdf (for top of window)
1 x 2 piece mdf (sides and bottom)
1 inch piece rounded edge mdf ( I couldn't find this at Home Depot, but local lumber store had it. Also for top of window )
Here is the 1 inch rounded piece
First measure and cut your 1 x 2 inch pieces to fit along the side of your windows like this. Verify that each side is the same. They should be, but my first window my measurements were off by 1/8 inch.
Pay no attention to the window clingy's :-)
I built the top piece before attaching any of it.
Measure the across the top of your window, including the width of the two side 1 x 2 pieces you already attached. Cut your 1 x 4 piece according to that measurement. Then, cut two pieces of your 1 inch rounded edge pieces to the same size.
Apply wood glue and sandwich the 1 x 4 piece in between the two 1 inch pieces like this.
I left it clamped for a few hours before attaching it.
Once dry, attach it to the wall above the window.
I used the existing ledge. I sanded, primed and painted it.
Measure from each side below the ledge and cut another 1 x 2 for the bottom below the ledge.
Apply a small amount of caulking around the window in the seems where the mdf meets the wall, and where the pieces of mdf join.
Finish with paint, and your done.
Ready for the before and after?
One last side by side comparison.