Normally, the new trim work would be completed after all other work. But in this case, the trim work is done first so that the log siding will butt against it. To begin the trim work, first measure inside dimensions of window. Measure the width and depth of the window sill and sides.
Cut 1 x 6 pine to length, the rip to depth needed. Set window sill into position first. Attach with 2-1/4" trim nails. Place the top sill in the window. Next, set the two sides in and wrap the interior of the window.
Measure the outer dimensions of the window, then cut decorative 1 x 4 fluted casing on the chop saw. Align with the window sill, leaving a 1/4" reveal, and nail in place.