Measure the sides of the window using a measuring tape. This is measurement A.
Measure across the top of the window using a measuring tape. This is measurement B.
Measure the width of your casing and multiply by 2. This is measurement C.
Cut two lengths of window casing that are equal to measurements A and C added together using a saw. These are the vertical pieces of your wood casing.
Cut two length of window casing equal to measurements B and C added together. This is the horizontal piece.
Bevel the ends of the wood casing to 45 degrees using a miter saw. This allows the strips of casing to fit together like a picture frame. When making the cuts, one side of the casing should end up shorter than the other side, rather than both sides being the same length.
Wipe the dust away from the wood using tack cloth.
Apply a thin layer of adhesive caulk to the flat back of the window casing.
Press the window casing to the edge of the window. The pieces should fit together smoothly at the corners and form a rectangle or square, framing your window.
Allow the adhesive caulk to cure completely. This takes around 24 hours.