diy window trim

Hello, friend! I’m so excited to share my DIY Window Trim Tutorial with you today! It’s been a minute since I’ve gotten a new DIY done! I was tired of doing small projects, so I decided to dive into a BIG one that I’ve been wanting to do for awhile – trimming out all of our downstairs windows!

This project took awhile, mostly because we have 8 windows, but the process itself is rather easy. It only requires a few basic tools (miter saw and jigsaw and preferably a brad nailer), but that’s it!

**This post may contain affiliate links – please read my Privacy Disclosure for more information.**


In the photo below you’ll see the trim in progress – even unfinished it added a TON to the space! So let’s get started so you can add some character to your bare windows, too! (P.S. This project came out to about $30-$40 per window, depending on the size. Not bad if you consider how expensive hiring something like this out would be)!

diy window trim

What You’ll Need (per window):

  • miter saw
  • jigsaw
  • 2 1″x2″ board, cut to the length of your window + 7″ (NOTE: I used common pine board for most of this project, but our Lowe’s did not have enough, so we also used MDF so we could avoid going back to the hardware store during the lovely COVID-19 pandemic, ya feel me?)
  • 3 1″x4″ boards, 2 cut to the height of your window (read more below) and 1 cut to the length of your window + 7″
  • 1 1″x6″ board cut to the length of your window + 7″
  • 1 1×8″ board (OR 1″x6″ depending on the depth of your windows – see more below) cut to the length of your window + 10″
  • drill (only needed for double windows)
  • brad nailer
  • caulk + caulk gun (I used DAP Dynaflex 230 white paintable caulk)
  • 2″ angled paint brush
  • 4″ foam roller
  • Primer (I used Zinsser 1-2-3 Bullseye Primer)
  • Paint of your choice (I used SW in Alabaster in satin finish)
  • 2″ brad nails
  • wood fill
  • painters tape
  • small paint brush for touch ups at the end
  • 120 grit sandpaper
  • tape measure
  • level

How To:

Step 1: Measure how deep your windows are. If they are between 3″-4.5″, you will want to use a 1″x6″ board as your sill. If they are 4.5″-6.5″ deep, you’ll want to use a 1″x8″ board as your sill.

diy window trim

Above is a handy diagram of all of the boards used and how they are installed.

Step 2: Remove any existing trim you might have. Now measure the length of your window and add 10 inches.

You will cut your 1×8″ to this length – your window length + 10″.

diy window trim

Step 3: Mark 5″ IN from each edge, and however many inches your window depth is DOWN. You are creating the windowsill that will slide into your existing window frame.

Step 4: Using your jigsaw, cut on your lines. Measure 3x, cut once, y’all!

diy window trim how to

Step 4A: If you have double windows like I do, you’ll need to carefully measure where your next inset needs to be cut out (in the middle of the board your just cut). From the edge of your new cut (see above), measure inwards from each side however long each of your windows is. Now measure DOWN the depth of your windows, connect your lines, and cut out. (If you are only framing out a single window, SKIP this step!)

PRO TIP: I use a drill to create holes at each corner so I can more easily manipulate and move my jigsaw! Here you can see how my two lines are connected by a line that runs parallel to the length of the wood. This parallel line is measured based on the DEPTH of my window.

Step 5: Sand your board well with 120 grit sandpaper. Install your windowsill into your window using 2″ brad nails and your brad nailer – or a hammer and nails if you don’t have the power version! It should fit snuggly into your window. However, if there are gaps, don’t worry! We’ll fix those with caulking later!

diy window trim

Step 6: Using a tape measure, measure UP from your newly installed windowsill to the top of your window on each side. I measure both sides because sometimes they are slightly different. Cut two 1″x4″ boards to the lengths you just measured. Sand with 120 grit sandpaper.

Step 7: Install these boards on the outside edges of your window with 2″ brad nails and your brad nailer (or hammer/nails). Make sure they are level before you install!

If you have a double window with a middle piece, you’ll need to figure out the width of wood you need there. For one of ours, a 1″x8″ fit perfectly. For the other, I had to piece together two 1″x4″ boards plus a 1″x3″ board. You’ll follow the same process as Step 6 to install this middle piece. Again, if you are only framing a single window, SKIP this paragraph!

diy window trim

Step 8: Now cut your two 1″x2″ boards and your 1″x6″ board to the length of your window plus 7″. Sand them as needed. Assemble your header on the floor, using A LOT of 2″ brad nails and brad nailer (this part is WAY easier using the power nailer – if using hammer and nails you’ll probably need 2 people). You will position the first 1″x2″ on its side, and then on top of your 1″x6″ board so one side is completely flush. In other words, the 1″x2″ board should be protruding out from the 1″x6″ board. CAREFULLY nail together. Repeat on the other side of the 1″x6″ board with your second 1″x2″.

diy window trim

Step 9: Now attach your header to the top of your window above the newly installed 1″x4″ boards. Use 2″ brad nails and use A LOT of them to make sure it’s secure. I suggest driving in one nail every 2-3 inches along the 1″X6″ board. Make sure your header is LEVEL and CENTERED before installing!

Step 10: Cut your last 1″x4″ board to the length of your windows + 7″. Sand as needed. Using 2″ nails, install this board directly underneath your windowsill – with the 1″ side abutting the sill. Again, make sure the board is LEVEL and CENTERED before installing.

diy window trim

Step 11: Using wood fill, fill all of your nail holes (or seams if you have them from any double windws). Allow to dry and then sand smooth.

Step 12: Using paintable caulk, caulk ALL of places where your wood meets your walls or where the wood joins each other at a 90″ angle/corner. Allow to dry.

Step 13: Using a small brush and paint that matches your current wall color, paint around all of the edges of your window where it meets the wall. I did this because the caulk tends to bleed a bit, so this will save you more touch ups at the end. Allow to dry.

Step 14: Using Painters Tape, tape off the exact same places you just painted – where the trim meets the outside walls. You can also tape off your actual windows if you choose.

Step 15: Using your angled paint brush, paint one coat of primer on your trim. and the inside of your window boxes (this will make the trim and windows look more seamless). Allow to dry.

Step 16: Using your angled brush for the corners/creases and a 4″ foam roller for the flat portions, paint your choice of paint onto your trim/inside of the window boxes. Repeat (2 coats will look much better, trust me)!

Step 17: Carefully remove your painters tape about 2 hours after your final coat has been rolled. Use a utility knife to score the tape if necessary. Once the tape has been removed, use a small paint brush and your wall color paint to do any last touch ups.

And that’s it! You’re done! There are a lot of steps, but it’s not a hard project! Just be patient and I promise it will be worth it in the end, because your windows will be FAB! And who doesn’t want FABULOUS windows?!

And just for fun, here a couple before and afters for you!

diy window trim

The biggest and most noticeable difference is in our kitchen because we don’t have curtains. Hubby wants me to get rid of the curtains in the living/family rooms, but I’m just not ready to part with them yet! Maybe someday!


We are smitten! It’s amazing how much brighter our downstairs looks too, just with a little extra white! Plus, the windows no longer look builder grade, but actually look custom and beautiful!

So tell me, have I convinced you to add this DIY window trim to your list? If so, please Pin to share! Until next time…



diy window trim tutorial


* indicates required