How to Install Window Trim

install classical window trim with crown molding in two windows in bedroom - teal walls

My bedroom makeover has finally reached that magical milestone where the "pretty things" start to arrive.

Yippee!

After months of hard work with very little visual payoff, the room has officially shifted from a chaotic construction zone to a space that says, "Hello, I'm a beautiful bedroom!"

The secret to this transformation?

Installing DIY window trim.

I've always believed that adding architectural interest is one of the most vital steps in any home renovation. I like to call these "structural details"—the elements that provide a room with the necessary weight, scale, and character. In this space, I've already installed faux wooden beams on the slanted ceiling, and I'm nearing the finish line on a large built-in unit.

However, as I stood back to admire the progress, I noticed the windows flanking the built-ins looked completely naked! 😳

Look at the "Before" below 👇

Aside from a tiny skirted sill, they had zero character. It was time to stop ignoring the "boring" and start adding the custom craftsmanship this room deserves.

If you've ever wondered how to level up a basic room, adding window molding is the ultimate game-changer.

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This is only a decorative treatment. All the materials can be found at your closest Home Depot.

The tools you'll need:

Miter box and saw (A Miter Saw will be even better)
Jig Saw
Sander
Nailer
Pry bar
Utility knife
Wood glue

How much material you need depends on the size of your window, but this is what I used:

detailed list of moldings used to update a window

DIRECTIONS:

1. DO THE MATH

Measure your windows. Write down those measurements. They're going to be helpful to calculate the amount of material you need to buy, and you can also use those measurements to plan your window trim design.

If you need to trim many windows, measure each and every one. Sometimes they look similar, but the numbers may differ.

2. REMOVE THE OLD WINDOW TRIM IF ANY

In my case, I had to get rid of the old window stool and apron.

Using a utility knife, I went around both pieces, cutting the caulking. This will prevent drywall rips.

using a utility knife to score the caulking around the window

Use a pry bar to lift the apron.

A wood shim will provide the necessary support and prevent damage to the wall.

Once the apron is off, you can begin lifting the stool, again, using the pry bar.

I found lots of wood shims underneath the sill that were used to level the stool. They were left in place.

Then, I went ahead and scraped and removed all the old caulking from the window and wall.

Don't throw away the old window stool just yet. You can use it to trace its silhouette onto your new material.

3. CUT & INSTALL THE WINDOW SILL OR STOOL

Once again, you need to measure to determine the length to cut the 1 x 5 to size.

This is how it worked for my windows:

Width of your window: ( ), in my case it was 32"
Add the width of both side casings: ( ), mine was 7" (two - 1x4s)
Add 1/4" on each side for reveals: ( 1/2")
Add 1" on each side for the "horns" ( 2").

The total length of the window stool I used was 41½."

Mark the centers on the old and new stool boards, align them, and trace the new pattern.

tracing the old window sill into the new material

If you are working on more than one window, it is a good idea to mark each board.

Even though they look the same, their measurements can vary.

Use the jigsaw to notch off those marked ends.

You can now install the new stool on the window.

Make sure it is level. If not, level it up with wood shims placed underneath.

Using a brad nailer with 2 1/2" nails, set it in place.

4. CUT & INSTALL THE APRON

The apron dimensions are the same as for the stool, BUT without the horns (2") and made out of 1 X 4 material. The horns are those extra parts on the window stool that go beyond the apron in this next picture.

Center the apron below the stool, level it, and nail it in place.

5. CUT & INSTALL THE SIDE CASINGS

You can find the length of the side casings by measuring the distance from the stool to the top of the window opening. Add 1/4" (top reveal line) to that measurement.

Set the side casings in place with brad nails.

6. INSTALL THE HEAD CASING

Once both side casings are installed, measure the distance from the outer, top points of both side casings.

However, a better way to determine the length of the head casing is by simply placing the 1X6 board on top of the side casings. Align it to one side and mark the ending point on the other side, as shown in the picture below.