How To Install Interior Window Sill Trim. Step 2 Remove the Trim. Remove the old window sill.
Nail the thick part of the trim to the framing with 2-in. Ours is 28 inches wide Then decide how wide you want your window sill to come out. Mark the location of each side casing on the wall.
Steps on How to Install an Interior Window Sill Mark 316 or a ΒΌ reveal mark on your window jambs.
Repeat for the opposite jamb. If the trim and surrounding woodwork feels cold to touch chances are that it is not properly insulated. Nail the trim to the jamb with 1-in. Measure between the marks add 2 inches and cut the stool to this length.