Attach the Trim
Once the pieces are cut, use a finish nailer or hammer and finish nails to attach the trim around the opening (Image 1).
A nail punch may be necessary to hammer in the finish nails far enough and not to damage the trim with the hammer.
Many styles of trim, like dental molding, have deep indentations that make hammering difficult but also hide the nail holes better on the finished product.
Fill the holes with wood putty, being careful to use as little as possible and smooth this with a putty knife or your finger (Image 2).